The only way to play GBA games at the moment is to buy a slot-2 card and use the R4's built in passme feature to boot to the slot-2 card, saving you a few dollars on the passme.
I'm wondering if he means that he has a DS and the R4 but wants to play original GBA games from slot 2. You can download a small .nds file for your R4 that will boot the GBA cartridge. I've had a quick look on the forum, but can't find the topic about it