Like Evil Rich said above me, if you do get a Guerrilla Air 48/45 make sure you get the European version (the grey version) - LIPS stock them at the moment.
http://www.lpspaintball.biz/cart.html
You won't be able to find them on the website, but I'm sure you can find their number and give them a ring and order via the phone!
From a 48/45 and a 4500psi fill, I'd expect at least 700/800 shots, and a maximum of 950, it depends on how efficient your marker is.
If you only fill to 3000psi, you'll probably get 500 to 700 shots, once again, it is very dependent on how efficient your marker is.
Those are just rough figures so don't take my word for it!
Out of curiosity, why are you going for the 48/45? And if you are concerned by the (possible lack of) air supply, go for a 68/45 or bigger!
It depends on your size though. I'm very short so the biggest I would go for is a 68/45, though I plan to get a 48/45 just so the tank doesn't get in my way.