Turning is a big concern. You'll want to make sure the track width is greater than the wheel base using 4CIMs and a reasonable gear ratio (10-12 ft/s). The greater that ratio is the less current your motors will draw when you turn, meaning you'll have better batteries for the end of the match (important for SCALING).
Track width is distance between the two sides of the robot as measured from the center of the wheel contact patches. Wheel base is measured from the rear-most to the front-most wheel, and usually you can easily do it by the center of the holes in the axle.
For example, say you want 15" tires, 4WD. Assume a 1" gap between the tires just for math's sake. We have then figure out how wide the drive base is allowed to be based upon the FRAME PERIMETER rules. If there is 1" from the front of the FRAME PERIMETER and the tire, and also 1" on the back, your total robot length is 33". Thus, the maximum allowed width of the FRAME PERIMETER is 27".
A cursory search for 15" tires shows the skinniest coming in around 4", but let's say you do bike tires that are about 2" wide at the tire. Using the KOP drive base, there will be about 1" between the edge of the FRAME PERIMETER and the edge of the wheel, then another inch from the edge of the wheel to the center of its contact patch (so the contact patch is 2" from either side).
The distance between the axles, or wheel base, is a wheel radius + 1" gap + wheel radius = 16".
The side-to-side distances between the contact patches is 27" - 2*2*, or 23".
Generally speaking, you'll probably be fine so long as you aren't geared to go super fast and the tires aren't super knobby. It's how Team 25 got away with 6 traction wheels on the floor for so long
My team has done 4 wheels every year since 2012. This may be the first year in a while we've done 6 wheels, but we haven't given up on the idea of 4 just yet...