Using 4 geared motors is commonly done on one dual ESC. Wire the motors in series so that they share the load. Remember that when you do this you have to double the battery voltage. So two 12v motors will need 24v to drive them, so two 12v gel cells will be required. This will drive all four motors ok because a dual ESC handles each side independently. Check out Vantec's dual ESC's and for cheap motors look up robotcombat.com's AME motor series at http://www.robotcombat.com/marketplace_motors_ame.html . A junkyard will have the cheapest wiper motors. You want about 125rpm as the final drive speed.
There's a great link on building a robot at http://www.coolrobots.com/builders/topframe.html . This is Christian Carlberg's site. He built Minion which was a champion battlebot on TV's Battlebots for a while. Man I really loved watching that bad boy go. Check out the motors in that thing! His main page is http://www.coolrobots.com/ .
Try putting robot parts into google and you'll kill a few hours easily.