After a bit more research, I found that the SD cards and SDHC, which are physically the same, but are two different protocols that are not interchangeable. Thus, a SDHC card will fit into a SD card device, like a reader, but will not be recognized by the computer. Apparently, you must use an SDHC reader.
Panasonic, one of the developers of the SD and SDHC spec., supports SDHC in the HPX2000.
Now I will be purchasing an SDHC card reader in order to use 4MB cards.
I hope this helps somebody else.