Ok. if you want your maximum speed to be 4 m/s (lets say that is at 90% of your free speed). it means, your robot will have maximum power at a speed of
(4m/s)/0.9/2 = 2.2 m/s
if your robot has 4 x 3000W motors, your total power is 12KW.
Since power = Force x Velocity, at 2.2 m/s, your robot can pull 5.45 KN which means it can lift~ 545 kilograms vertically.
Here is where the issue is. The maximum coefficient of friction/traction that you are can dream of getting is lets say 1.5 Which means you are only going to be able to pull 60kg and the rest of the power is wasted.
Thus your usable power is: 60 * 10 * 2.2 = 1320 watts. or 330 watts per motor.
I am guessing you have not been involved in FRC. a CIM motor with a Jaguar speed controller on each wheel will give you the necessary power to accomplish your task.
motor controller:
http://parts.digikey.com/1/parts/180...mdl-bdc24.html
motor :
http://www.andymark.com/product-p/am-0255.htm
you also need a gearbox for that. with 15cm diameter wheels, you would need an 8:1 transmission found here:
http://www.andymark.com/product-p/am-0145.htm
modified with a 19tooth and a 45 tooth gear.
Hopefully I was able to help.
Also here is a robot we build in 2010. I am sure it would be able to accomplish your task. with some minor modifications
http://www.youtube.com/watch?v=LVtHI6fpeGM