Two points: none of the locations where you can get a GF are closed in the 4th disc; this includes Centra Ruins (Tonberry), Deep Sea Research Lab (Bahamut), Tear's Point (Doomtrain), Tomb of the Unknown King (Brothers). This means that, by drawing GF's from the bosses in Ulti's Castle, it's possible to obtain all GFs in disc 4 (except maybe Diablos, if you didn't get the magic lamp from Cid).

Second: I've not tested this, but IN THEORY, it would be possible to get Tonberry without setting foot in Centra Ruins. Tonberries can be encountered in the corridor that leads to Ultimecia's chamber, so if you ran around there, and fought enough tonberries, you could get up to 20. Whether or not TK would appear, I don't know.