I bought one from grassroots. popped it in the phone, but "storage card" file didn't show up (unlike my SDHC, which just hangs the phone until you pop it out). formatted, but got I/O errors when copying to it initially. I persisted, and after 2 or 3 I/O errors it started writing fine (a little slow tho). just did a few chunks at a time, now I have ~3.6gb and it seems to work fine so far. I'll keep you updated.
oddly enough, when I first started copying files, I was actually putting them on my old 2gb, which was already full. I got up to 2.5gb before I realized what I was doing.
I would like to hear successes & failures from other grassroots users. obviously there are some 4gbs that work & some that don't &/or are fake. I haven't heard failures on these from that particular supplier tho.