I'm assuming you're quoting what's written on the expansion board. It's probably capable of holding 8Mb worth of SIMMs, but has only 4Mb installed. It may or may not have an FPU installed either. If there are no free SIMM slots, you can only upgrade to 8Mb by replacing the existing SIMM. On most boards, however, upgrading to 8Mb will prevent you using the PCMCIA slot.
