AIX

There is problem in my system whenever i run a command lsattr -El mem0 it shows 3gb memory and when i run lscfg -vp | grep Size it shows 8 modules of 512 mb. it means it shows 4 gb how is it possible.

there are a few reasons you will have a diffrence in memory
#
lscfg -vp list all mem dim's
lsattr -El mem0 list avalable mem to the system OS

POWER5-based systems use memory for the Hypervisor
On a lpar system the Hypervisor will take some of the memory and use it in all modes.

Missing or faulty memory
If in any doubt , then you might want to just go into Service Processor Menus (hit F1 key for graphical console) to see if all the memory DIMMs are enabled.