Centos 64 bit only seeing 3834MB Ram

Hi
i have installed Centos 64 bit on Windows 2008 Hyper -V.

guest has 5012 Ram

so centos should see 5GB of ram.

but its only see 3834 mb Ram !!


some info :

free -m
             total       used       free     shared    buffers     cached
Mem:          3834         89       3745          0         13         35
-/+ buffers/cache:         40       3794
Swap:         2943          0       2943
[root@sandbox ~]#
[root@sandbox ~]# uname -a
Linux sandbox. 2.6.18-238.19.1.el5 #1 SMP Fri Jul 15 07:31:24 EDT 2011 x86_64 x86_64 x86_64 GNU/Linux



but
when i boot the same vm with Debian live 64 bit, its see full memmory

I never seen this problem before

can any one help me with this

thanks