The EZ4 has a GBA loader and a DS inbuilt. Both load GBA roms but the DS loader can load DS code (for the few bits of older homebrew that do not work on DS slot devices) and use the touchscreen.

The GBA will boot on a GBA menu or the load gba option on the menu of the DS.

The DS loader will appear if you have flashme installed or you use the nopass options of the M3 simply (the EZPass is a passme 1, the EZPass2 is a passme2 and the EZPass3 is the EZTeam's nopass but I doubt you will find the first two for sale these days so you should not have to worry about that).