Detecting 4GB RAM on a 32-bit Ubuntu server?...
Generic kernel in 32-bit Ubuntu machine won’t detect 4GB RAM. It can detect only 3.2 or upto 3.5GB RAM. Upgrading generic kernel to server kernel will help you to use full 4GB RAM. The command below will help you to install server kernel and then reboot the server.
# apt-get install linux-server linux-headers-server
Make sure PAE (Physical Address Extension) support is enabled in the server.