GTX 750 Ti 2GB - (PC Memory 4GB, but only 2GB usable)

Hi, need a helping hand here!

Just 2 days ago I upgraded my 5-year old PC to the MSI GTX 750 Ti 2GB. My PC has 4GB RAM. Before installing the graphics card, I can used the 4GB RAM, but somehow after installation, only 2GB is usable.

My motherboard has ...