Going on what you've listed and what you'd like, here's a possible option:

The speedos you've linked to don't have a high enough rating to run two drill motors each, so 4-wheel drive is out of the question. But what you can do is put 4 wheels on your robot but only connect two of them to drill motors. If you connect the two diagonal ones to drill motors you will have something that is not too different from full 4-wheel drive in terms of handling but which only requires two drive motors. That way, you can keep the cost low with the speed controllers and, if you buy four drill motors, you'll have two spare motors.

Are speedos really necessary you ask? Not at all. The full-pelt or nothing method of using micro switches operated by servos to control your robot will do the job. Not the nicest job, but they will allow you a degree of control over your machine. Driving 2-wheel drive robots with switches can be difficult but with a 4-wheel diagonal drive setup it is a bit easier, as the two idling wheels give a little bit of friction to prevent the robot from spinning wildly when you turn and also help to keep it going in a straight line.

Ancient video time!!
This video illustrates the idea mentioned above. It's got four wheels (well, was meant to, but only has three, lost the other one ) but only the two diagonal wheels are driven by drill motors. Control is provided by micro switches operated by RC servos:

http://s31.photobucket.com/albums/c367/ ... t_0001.flv