I decided to use a fairly common chassis from feetechrc (www.feetechrc.com) FT-MC-003 available from several distributors as it provides the four continuous rotation servo motors as well as the wheels and rubber types, plus the chassis which all fit together nicely, albeit a bit fiddly. The photograph should show the chassis with the Arduino Nano mounted onto a piece of strip board to allow connectors to be used instead of soldering the motors. I used a Nano as it is low cost and has everything needed (at the moment anyway). Power is provided by the 4 x AA battery pack which also has an ON/OFF switch. I am only using the motors in full speed mode(forwards and backwards) at the moment. It takes a bit of fiddling around to get all the motors moving in the correct directions as none of the datasheets seem to show which direction the motors rotate for different pulse widths.
The YouTube video at https://www.youtube.com/watch?v=N_mAkRxrT_k&feature=youtu.be shows the chassis just moving forwards and backwards for 1 second periods. When the chassis changes from backwards to forwards it rears up due to the momentum and the centre of gravity being nearer the back due to the battery pack position. It might be necessary to change the battery pack position if this becomes a problem. At the moment any problems will be overcome via software. There is some problem as can be seen in the YouTube video that after a few cycles it suddenly stops and then restarts. This isn't programmed so I think that this is a power dip caused by motor in-rush currents when changing direction that is resetting the Nano. I hope to fix this in the software otherwise a separate power pack for the Nano might be needed. At the end of the YouTube you can see the advantage of four wheel drive as the chassis climbs over my obstacle strewn desk without any problems.