I could be wrong but this is my understanding: GRP0-3 is simply for you to PUT the sprites or draw stuff (like the memo mode in my game). The default top screen uses GRP0 and touch screen uses GRP1. But in order to put your sprites, the sheet has to be stored somewhere first, and that's GRP4. So you load your customized sheet to GRP4, then SPSET to GRP0 (top screen default so no need to type that)