Question: The gemorah says R.Bibi took 400 zuz for his daugther. Other places in shas the 400 zuz for a dowry comes up. What is the significance to the number 400? I once heard it refered to a type of tuma. Is there a referance to 400 being a specific type of tuma? Lastly isn't the dowry for a besula 200 zuz?
(a) It is not exactly clear what this 400 Zuz here was and who received it. Did Rav Bibi receive it in return for giving his daughter in marriage, or did the daughter herself receive it? There are conflicting Girsa'os in Rashi; our Girsa says that she received it, but the emendation at the side says that he received it.
If she received it, then it was the money of her Kesubah. We find that 400 Zuz is given as the Kesubah to women of great merit ("Ishah Chashuvah"); see for example Kesuvos 12a and 99a. This amount in particular is given because it is double the normal value of a Kesubah (200 Zuz).
(b) I do not know of the number 400 referring to a type of Tum'ah. (There is a reference to the number 300 (the "Shin") being connected to some type of Tum'ah.)
See the KLI YAKAR (Bereishis 23:16) regarding Avraham Avinu's purchase of Ma'aras ha'Machpelah for an elaboration of the significance of the number 400; it is not clear, though, whether the Kli Yakar's approach will apply here.
(c) The Kesubah for a Besulah is 200 Zuz, although it can change depending on the woman (see above (a)). A dowry , on the other hand, is not fixed (usually a dowry refers to what the father gives the groom, and not what the groom gives the father).