To see what this is all about, read Part 1, Part 2, and Part 3 first, then come back here.
Ok, did you read them? Good. (For those who have been around from the beginning, note that I have added two paragraphs to Part 2, which I had erroneously deleted before posting the original post. They're at the end of the "kesef" section.)
Part 4 is about the ketubah. Before getting into what a ketubah is, we'll start with what it is not:
A ketubah is not the same as a sh'tar kiddushin. This distinction is discussed in Part 3. Some ketubot serve as (among other things) a written record of kiddushin, and include the text "X said to Y: 'You are hereby consecrated to me...'", but this kiddushin takes place whether or not it is reported in the ketubah.
A ketubah is not a document that enacts marriage. Two people who get married without a ketubah are still married by any definition. In order to make husbands give their wives ketubot, the rabbis ruled that a couple without a ketubah is not permitted to live together; however, they are still married.
A ketubah is not, classically, a document under which a woman is acquired for the price of 200 zuz. As discussed in Part 2, insofar as classical Jewish marriage resembles acquisition, the price is actually (for better or for worse) one perutah! And once again, if there is anything resembling acquisition, the ketubah has no part in this.
Ok, so if a ketubah isn't any of the things that it is commonly believed to be, then what is it?
The classical ketubah was essentially a "severance package". The husband would give the wife this document at the time of the marriage, and then if the marriage terminated, due either to divorce or the husband's death, the wife could cash in the ketubah and collect the designated amount from either the husband or his estate. (In those days, wives did not automatically inherit from their husbands; the inheritance would go to the offspring or closest blood relative.)
The ketubah was not in itself a misogynistic document. Rather, given the context of a society that would be considered misogynistic by today's standards, in which most women were not independent economic actors (and any income earned by married women went directly to their husbands), the ketubah provided a corrective by ensuring that divorced or widowed women would not be penniless when their marriages ended.
The minimum amount granted in the ketubah was, generally speaking, 200 zuz for women who had not been previously married, and 100 zuz for women who had. There was no maximum amount; any amount above the minimum could be specified in the ketubah. This 200 vs. 100 distinction makes some amount of sense: a woman who had been previously married (and therefore had previously collected at least one ketubah) might already have some financial assets of her own, and therefore be in less dire need than someone who had gone directly from her father's household to her husband's household and had nothing to her name. (But categories such as mukat eitz certainly problematize this rational basis for the distinction.)
How much was 200 zuz? Well, according to a popular song, two zuzim was the price of a kid (a goat, not a child), which means that this was enough to buy 100 kids. But more seriously, in a source more contemporary with the original ketubah, the Mishnah (Peah 8:8) defines 200 zuz as the poverty line: someone who has 200 zuz is not eligible to collect funds designated for the poor. So this is a significant amount, intended to put the divorced or widowed woman on her feet so that she's not living in poverty.
Furthermore, the rabbis ruled (Mishnah Ketubot 4:7) that a woman who never received a written ketubah was still entitled to collect the appropriate minimum amount (100 or 200 zuz) on the termination of her marriage; thus, this provision of the ketubah became automatic, with or without the actual ketubah. This chapter of the Mishnah goes on to name other ketubah provisions that go into effect and are enforceable whether or not they are actually written into the ketubah: the ketubah obligation is secured by the husband's property; if the wife is taken captive, the husband is responsible for ransoming her and bringing her back; if the wife dies first, her sons can collect the ketubah amount on top of what would otherwise be their share of the inheritance (this was more relevant in a time when men had multiple wives, so there were multiple sets of sons competing for the inheritance); the wife's unmarried daughters will be supported by the husband's estate after he dies; the wife herself will be supported by the estate until she is paid her ketubah. We can infer that these were all meant to be written into the ketubah. Most of these provisions, like the original core of the ketubah, deal with responsibilities that accrue after the termination of the marriage. The one exception is the part about redeeming the captive, which is a responsibility during the marriage, and is therefore perhaps the link from the ancient to the modern ketubah.
Over time, the ketubah grew to incorporate more responsibilities during the marriage, and the standard wording (see, e.g., Rambam, Hilchot Yibum v'Chalitzah 4:33) came to include אנא אפלח ואוקיר ואיזון ואפרנס יתיכי ("I will serve and respect and sustain and support you"). Thus the current "traditional" ketubah text contains both some of the responsibilities of the husband during the marriage and the financial guarantee after the termination of the marriage.
Modern egalitarian ketubot tend to focus on responsibilities during the marriage, though of course, the responsibilities apply to both partners. Our ketubah is in this model, containing both the responsibilities (in Aramaic) found in classical ketubot and some (in Hebrew) based on the text of Rachel Adler's berit ahuvim and other versions used by our friends. All provisions are mutual and incumbent equally on both partners; adapting the text for a same-sex marriage would require only grammatical changes. We intentionally used very little original text, and the two couples working on this used identical texts (except for names, dates, and locations), in accordance with the principle (discussed in Part 1) of doing things in a general way that other couples could use rather than specifically tailoring the ceremony to ourselves.
As for the original core of the ketubah, we included a paragraph stating that property would be under joint ownership, and a paragraph regarding the termination of the marriage (which will be discussed in Part 5). However, there is no reference to 200 zuz or any analogous amount, because we couldn't see any coherent way to incorporate this into an egalitarian ketubah between two partners who are both financially empowered. It wouldn't have made sense to apply this financial responsibility to both partners: in our society, it is assumed that spouses inherit each other anyway, so specifying a fixed amount payable to the surviving spouse would have been redundant. And in the case of divorce (in which both partners survive), the two payments would cancel each other out and would thus be meaningless. Instead, in the case of divorce, we included a requirement that כל רכוש דאית לון יהון פלגין באורח קשוט וכל טפלא דיתברכון בהון יהו באחריות תריהון ("they will divide all of their possessions with integrity and share responsibility for any children with which they are blessed"), which we think matches the original intent of the ketubah, to ensure that both spouses and any children are provided for after the termination of the marriage.
So that's the text of the document. The other issue is the means of accepting it.
Classically, ketubot were signed only by the witnesses. Nowadays, many ketubot are also signed by the couple (and/or the officiant(s)). We went with the former approach, but not for any ideological reason.
At many weddings, the couple (in the case of an egalitarian ketubah) or the groom (in the case of a non-egalitarian ketubah) formally acquire the responsibilities of the ketubah via the mechanism of kinyan sudar: They take hold of some object, such as a handkerchief (sudar) or pen, that belongs to someone else (e.g. one of the witnesses), and in taking possession of that object, they acquire the ketubah obligations. The witnesses watch this happen, and then sign the ketubah to affirm that they have seen it. Typically this takes place before the chuppah. (However, it occurs to me that couples that don't do a formal kinyan sudar are still covered if they sign their own ketubah, since in 21st-century America, signing a document is the standard method of accepting responsibility for the contents.)
You may recall from Part 3 that we didn't use rings for kiddushin. But we still wanted to exchange rings. So we used the rings as the "sudar" to acquire the obligations of the ketubah. (This is similar to R. Dov Linzer's model for a bilateral nonegalitarian "double-ring" ceremony, in which the groom gives the bride a ring to effect kiddushin, and the bride gives the groom a ring which he accepts to take on the obligations of the ketubah. However, instead of 1 ring for kiddushin and 1 ring for the ketubah, we used 0 for kiddushin and 2 for the ketubah.)
We gave the rings ahead of time to one of our witnesses, who was then the owner of the rings for a brief period of time. At the appropriate moment (after the ketubah was read aloud under the chuppah), she brought the rings forward to the chuppah in a bag. We then lifted the bag of rings together, to indicate our intent to establish a partnership. (This echoes the berit ahuvim, which is in turn based on the classic method of establishing a business partnership (shutafut). However, we were doing this only symbolically, since shutafut requires pooling resources that belong to each partner, whereas the rings still belonged to our witness at that point.)
We then removed the rings (which still belonged to our witness until then) from the bag, and each of us gave the other a ring. In taking possession of the ring, we each accepted responsibility for our respective obligations in the ketubah. The witnesses witnessed this, and signed the ketubah. (This method required signing the ketubah under the chuppah, rather than ahead of time, since the witnesses were testifying that they had seen us accept the responsibilities.)
This was followed by sheva berachot, etc., as at any Jewish wedding.
Coming in Part 5: provisions for divorce