If you search for the address 400004Ch on https://problemkaputt.de/gbatek.htm https://problemkaputt.de/gbatek.htm it leads to the mosaic control register. It do

If you search for the address 400004Ch on https://problemkaputt.de/gbatek.htm https://problemkaputt.de/gbatek.htm it leads to the mosaic control register. It does a primitive pixellation effect (no averaging or anything, it just holds the top left pixel of the region) on a background layer. The exact same effect is present in the SNES and DS and is often used for level transitions. I don't know enough Z80 to know how you use the API call though.