Hey cool, I got mentioned! Here's the deal with crab: This year is a wonderful year for it. if you drive each wheel independently, you can do some pretty awesome things with traction control. In addition, your module can be pretty minimalistic due to the traction. Your robot will slide before most modules will bend, which allows for some pretty simple designs. If you've got access to a Manual Mill, I can give you some tips on how to build a pretty simple drab system.

Also, someone mentioned just doing 2 wheel crab. This is a decent idea, but consider this instead: If 4 modules are out of your reach, 2 will be also. The machining difference between 2 and 4 is minimal; most of the work is in the setup. Instead, why not go with an Ackerman style steering on the front, which a Differential in the back? With some nifty programming (which we have more than enough power to do), it should be pretty simple to get some Yaw control going, much akin to the current Subaru STI and the Mitsubishi Evo. Power your rear wheels at differing levels while steering your front, and trust me, you'll be impressed at the performance you'll be able to achieve.