A 4 or 4.5" "knockout punch" is unlikely to fit the budget, they're expensive used and extremely expensive new.
http://www.ebay.com/itm/Greenlee-Kn...MM-4-1-2-500-4654-500-5644-/151250032858

http://www.amazon.com/Greenlee-742BB-Standard-Knockout-2-Inch/dp/B0020448TI