I have also been using a 4 gig for about as long as we've known the module could be imported. I might hve been among the first to try it, in fact. that isn't the point.
try this:
backup your 4 gig sd
fdisk it to type 6 (windows FAT32..or was it type b--I always have to check)
format it vfat -F 32
check the size
I bet it will say 1 gig
type 82/ext2 works just fine.
this is actually a known issue.
I had originally been mildly curious as to whether anyone knew a fix, not intending to inform you of a defect. I think the problem maybe has something to do with the module, since the kernel can see >4 gig FAT32 on other media, and since the issue doesnt exist in 2.6 based roms. I seem to remember hrw saying it had something to do with block sizes.
Have you really never run into this? there was a thread running about it last fall.