Supermicro P3TDDE : 4GB memory installed, but only see 3.6GB on POST

[Matthew Lee]

my motherboard is P3TDDE, for dual PIII-S tualatin CPU. i installed 4*1G SDRAM memory stick.
first 3 were identified successfully, the last one plugged in, and only get 600M more.
i head some articles mentioned 'memory hole' or memory remap. but i cannot find them from the bios menu.
can anyone have any ideas ?