What is the point of this whole section? We have already had the same discussion about three! If the point of difference is that here the wives take a Shevu'a, then why not in the case of three? What is the point of the Shavu'a anyway? If it is only to aerradicate a possible suspicion among the wives, Is this not debasing the solemnity and seriousness of a vow - especially as it was in former times?
Ralph , Israel
The first Mishna (as explained in Rashi) is discussing a case where all of the Kesuvos were dated on the same day, therefore none of the wives have an earlier claim on the other. However, since in this Mishna the first wife may collect the entire estate, leaving the second wife with nothing, therefore the second wife can demand a Shevu'ah from the first. (Tosfos explains why it was necessary for the Mishnah to explain this Halachah specifically with four wives, and why two or three would not suffice.)
As for what you refer to as "debasing the solemnity and seriousness of a vow" -- it is precisely for cases such as this that Shevu'os were enacted. It is a Mitzvas Aseh Min ha'Torah to swear truthfully when necessary. There are many Shevu'os which the Chachamim enacted to allay the suspicions of others that might arise when a person collects money. (For a basic list, see Mishnah Shavu'os 44b.) These Shavuos were enacted in cases where there is some reason to believe that the claim being presented is not legitimate. The Shevu'ah protects us from honoring false claims which cannot be disproven in court.
D. Zupnik