DS: 100 million
PSN: a portion of PS3/PSP users who have internet access, and the smaller portion who use it
PSP: 46 million
FF4 was a huge success on the DS. Why would they move it to a smaller userbase, just so they could add glossier graphics that would result in higher production costs? That's a lose/lose situation. More expensive game with fewer potential customers.
I'm certain they'll finish the first 6 FF remakes on Nintendo hardware, just as they were originally released.
Besides, FF4DS was released on a 128 MB game card. 5 and 6 could go up to the newer 256 MB game cards if necessary.