I was told that the 4gb cards don't fit. I bought 2gb cards but the drawback is that I would have to change them if I wanted to use them both. From my understanding, the blackberry, the competition to the device I am trying holds 4gb if you really need it.
Although, from my experience, I already have a lot of crap on there and I just put my storage card worth 2gb in today and haven't had any memory troubles..
Good luck.