If running multiple applications and VM Fusion would I benefit (at all) from upgrading a UMBP to 8gb? Honestly, I'm coming from a PC and I'm not sure how to read the RAM usage on a mac.
Yes. If you plan on running Fusion and on top of that multiple apps, you would better off maxing out the RAM for your MacBook Pro. RAM is just as, if not, more critical than just the CPU.
so what is it with macbook pro 15.4" ? is it possible to have 8 gb ram on it or is it chip restricted to have that kind of quantity on it ? If there are two slots of same size like in mbp 17" and with same bus 1066 Mhz speed, how there are no upgrading posibilities on apple home page?