Welcome, Newcomer. You can use as many RAM sticks/modules as there are DIMM slots on the motherboard. So, if there are 4 slot, yes, you can use four modules. Generally it is better practice to match your RAM. This is to avoid the headache of adjusting the RAM settings in your BIOS to find what the mixed pair/set likes. However, it is NOT a requirement.
Whether or not all RAM will be useable is a different story. The 32-bit OS and applications will only use up to 4GB of RAM; the 64-bit OS and apps will much more. In a typical PC environment, you'd be fine using 4-6GB on a 64-bit architecture.