The shiur for 4 cups of wine

I'm learning through the gemaras, and trying to figure out how (and if) the halacha came to be a reviit per cup nowadays, rather than 1/4 of a reviit of actual wine per cup, mixed with enough water to make it taste ok.

Update: I learned the sugyot some more, and updated the web page, going through all the sugyot and showing how it is possible to learn them retaining the idea of a minimum total shiur of a reviit per cup. Still more to be looked at. Not to be relied upon halacha limaaseh.

Update: It is down for now while I make a number of major additions and structural changes.