So as a lot of you know I am making UndertaleSB™(not realy ™) and I need to use PetitModem and it goes to GRP0 and I need to use GRP4 or BG. Does anyone know how to copy GRP0s to GRP4s?

Use LOAD "GRP[GRP page]:filename" when loading.

No I mean if I have a GRP0 and I want to load it into a GRP4 or BG slot.

There is no part of a GRP file that forces it to be on a specific page. If that's not what you meant, then maybe use GPAGE and GCOPY.

Like V360 is saying, all a GRP file really is is a data (DAT) file, so it can actually be used on any page, regardless of which page it was on when it was saved. The page number is only specified when saving so SmileBASIC knows which page to use as the source for the saved file.

For example, although it was intended to be a SPRITE file, LOAD"GRP2:SYS/DEFSP.GRP" works perfectly fine and will load the default sprite sheet into the second GRP page.