Memory 64bit see 8gigs but can only use 4gigs

Fri Sep 30 22:06:59 UTC 2005

 kernel: Memory: 3992156k/8388608k available (2242k kernel code, 0k
reserved, 1322k data, 224k init)

             total       used       free     shared    buffers    
cached
Mem:       3993352    2541436    1451916          0     526208    
481032
-/+ buffers/cache:    1534196    2459156
Swap:            0          0          0

 2.6.13-1.1526_FC4smp #1 SMP Wed Sep 28 19:28:24 EDT 2005 x86_64 x86_64
x86_64 GNU/Linux

Any idea why I can see all 8gigs but can only use 4gigs?