Stockpiling T4 Materia for 4.0 is not really a bad idea.

Personally, I'm still collecting RGS because I'm outfitting my retainers in the i200 gear. Accessories (belt, right side gear) I'm crafting and melding i190 gear - shoving a pair each of T4 Perception and T4 Gathering Materia to max those stats for the servants. Working RGS will be a somewhat slow, but steady, replacement for those T4 Materia after I'm done re-outfitting them.