Will 2 2" drains drain a 4500 GPH pump? Yes, without a doubt. Will ONE 2" drain drain a 4500 GPH pump? No...I doubt it....well, maybe....I wouldn't try it. It's poor design.
Now, are you talking laminar flow--no turbulence, no air in the pipes, and no noise? Or are you talking turbulent flow. Pipes drain MUCH LESS with air sucking in, turbulence, etc. So when you ask if a "X inch pipe can drain X GPH", it depends on whether you're talking about turbulent flow or laminar. Sucking air, or full siphon? Makes a HUGE difference.
What I would do if I were you, is put PLENTY of drains in the tank, big ones. Enough drains that you just KNOW you can drain whatever size pump you may ever want to put on the tank. That way, you know. I have 4 2 inch drain holes in my tank, and then 2 1.5 inch holes for return lines from the pump--so I KNOW. (By return lines, I mean, the lines coming from the pump that push water back up through the overflow tower back into the tank from the sump.)
I'll tell you this, though--if you're drilling these holes for the drains yourself, you better be real sure. It's easy to drill a hole, no harder to drill a 2 inch hole than a 1 inch hole. BUT..if you drill a 1 inch hole, and later decide the hole needs to be 1.5 or 2 inch, that's hard to do. If you have 2 holes side by side, you're screwed because if you DO make the hole bigger, your bulkhead flange won't fit.
You need to get sure about all this...
Here's some links about all this....
http://flexpvc.com/WaterFlowBasedOnPipeSize.shtml
http://www.thereeftank.com/forums/f217/how-much-can-a-1-bulkhead-flow-129841.html
http://www.reefcentral.com/calc/drain.php
You gonna put overflow towers inside the tank? What kind? Coast to coast, plain vertical ones? What size?