This is apparently a common misconception. Technically, SD cards that follow the SD specs can only be 2GB in size. The SDHC spec supports cards larger than 2GB, but is more recent and isn't supported by many devices. Before the SDHC spec was finalized, there were a few manufacturers that tried workarounds to produce 4GB SD cards, but these don't follow specs and don't work in all devices. What I'm really saying is that at this point, all SD cards >=4GB should be SDHC cards.

(Apparently the original SDHC spec started out at 8GB capacity, so there really wasn't supposed to be 4GB cards. The spec has apparently been revised, or the manufacturers figured out another workaround.)