Okay, if I understood your post correctly, it is like this... Two-Four players connect their GBAs to the GC, and when all four players are together, everyone is displayed on the television screen. When a single player enters a building or cave or something, this is when the GBA comes into play, as the view for that player switches to the GBA Screen instead of the TV. You can also use the GBA screen to view secret messages only intended for you, and the like.
The gameplay is supposedly similar to the GBA Four Swords, but with the changes I mentioned above. It uses a GC Disc, which will make the game a lot bigger and so you can do more things at once.
FS for GC has received a very mixed opinion... It is a bit of a problem for me, because, as you pointed out, not many poeple around here have 4 GBAs with GBA>GC Link cables.