No. You are running a 32 bit operating system. With the architecture of 32 bit operating systems you CANNOT exceed its barrier due to its design. The barrier I speak of is normally 3gb or sometimes upwards of 3.5gb for whatever reason. But you will not be able to use all 4gb of your RAM. It's there, it's not "dead", it's just not usable.
If you install a 64 bit operating system on that computer, it'll utilize all 4gb of RAM. I wouldn't recommend doing this because there's drivers specifically for 32 bit and drivers specifically for 64 bit. If drivers exist for ALL of your hardware components for 64 bit and you don't mind dishing out some bucks, you can upgrade to a 64 bit OS. But otherwise, stick with 32. 3gb of RAM will make even Vista run pretty smooth.
Just curious, what kind of computer is this? Did you buy it from Dell or something and upgrade to 4gb of RAM and get stuck with a 32 bit OS? If so, I'd be calling them up and demand they fix the issue at hand. If you originally paid for less than 4gb, but upgraded on your own, well... not much you can do.