boss bv8962 is not reading sd card?

So, my friend has a Boss BV8962. He's got a 2 gig SD stick that it reads fine (at least it seems like it does to me). So, he wanted to put like 1000 songs on an SD card, so he went up to Frys electronics and bought a 4 gig SD card. Well his BossBV8962 won't read the 4 gig. Still reads the 2 gig and plays music from the 2 gig... But won't do anything or even load the 4 gig. The 4 gig works on a camera and all the computers we can possibly plug up to it... Just not the indash. I've read posts about this all over, but different head units have different reasons, so I thought I'd make my own topic. Anyone have experience with this? Thanks, Chris.

Oh for what it's worth, it's a 04? Impala... It's not overheating like I've read in other posts.