Have you only SEEN your son build a computer once, or has he actually only built one computer? I don't know your son, but if he's anything like me, he'd actually hate receiving a pre-built computer as a present. Even a nice one.
Here's what I think you should do- tell your son that you could not afford a birthday present this year, because money is so tight. Tell him you only got him a card. Give him a birthday card with a $4,000 check in it, and tell him to use it all to build his dream computer.
If your son is anything like me, having $4,000 to pick out and spend on his own parts would be much, much more awesome than buying him a pre-built computer. Like I said, I don't know if your son only built a computer once to try it, or if he really enjoys picking out and assembling his own hardware as I do. If you think he's anything like me, you should go with my idea!
Could do this.. but I already bought the monitor from amazon, unless there is a way to cancel it. But it's hard to give my son 4,000$ of cash... I don't know what he will do with all of it. He has good morals, so I trust him. But just the earier feeling of wasting it on something scares me.