I think there are a few varients of the r4 but the one I got supports up to 2gb only. It states no 'SDHC' (secure digital high capacity) which is generally anything above 2. But, keep in mind 2gb is about 40 games...if you want the r4 realhotstuff.com has it for $22 with a 2gb card included which is a rediculously good deal to me.
Cofree is right tho I believe the manual states 4gb but no SDHC and as far as I know, SDHC is just a generic term for anything bigger than 2gb.
I think mine is generation 1 and I think theres newer versions of the r4 but in my experience it's paid to get the cheapest flash card available
