Hello and welcome to the forums
You mentioned that you when you boot up with 3GB,it only reads 3GB.I guess you meant you install 4GB but your system reads 3GB of it right? If so,then that's because of your OS.
32bit OS's recognize max 3.2GB of RAM installed.You have to install a 64bit OS so that all the 4GB RAM becomes usable.
Actually, the 32-bit interface allows for up to 4GB to be read. That is, in theory -> 2^32 =~ 4.2Billion, which is around 4GB. BUT because of funny ways to split the memory in the specific 32-bit OS means only around 3GB will be available... Bummer, but that's life!