Do you have a retail GBA cart? Does it work in your DS? If not, the problem is your DS.
If GBA carts do work in your DS then obviously it's the EZ4, I'd give the contacts on the the cart a good scrubbing w/ a soft eraser and try again... The ones that connect to the DSÂ mean, not the SD ones.
Also make sure that the cart is assembled properly and it is connecting properly w/ the DSL.
Even if the SD were the problem the EZ4 should still boot up and complain that it can't find it and if you aren't using passme/flashme/etc it would still boot in GBA mode.
If that fails send the cart back for a replacement, it possible that it's deffective.