I noticed in another thread that you purchased your Kingston SD Card from Ebay. I'm not sure about memory cards, but for electronics in general I would be suspicious of such items you got from there (especially for things that can be made in bulk or if you found "a deal too good to be true"). The possibility emerges that your card can be counterfeit (not made by Kingston), but I would not know how to check for sure.
At the very least, if you got a retail package with the card (or assuming it's really made by Kingston), you can try and RMA the card to get it replaced by Kingston. Their flash memory cards, among other things, are covered by a lifetime
warranty.
Oh, and although I do not use a 4GB card (I'm don't feel like risking the few $ for all that extra storage I won't use), I do use a 2GB PQI MicroSD (w/ SD Adpater) in my Axim without any problems.