My mac has 4gb of memory but with nothing running it only has 3gb why

[mamama]

my mac should have 4 gb of memory but even with nothing running im only getting at most 3gb. any ideas?

 

[mikeangs2004]

You have 32 bit OS which can not recognize more than 3-4Gb of RAM. The OS might have reserved 1Gb for internal system use. If your hardware supports 64-bit you can upgrade it...