I don't mean to toot my own horn per-se, but there is a
tool available that shows the tradeoffs for 4-vs-2 motors with respect to top speed and current draw.
Attached are screenshots of how I'd design a 2-motor drive and the respective 4-motor equivalent (all else equal). It uses the 2011 KOP transmission with 6" skyway wheels. The exact #'s are always subject to verification (efficiency measurements, exact motor specs, weight, etc), yet it does show a meaningful difference between the two.
Note that the difference in max speed will be noticeable if the robot ever has to traverse the whole field. Other than that, if the 'zones of play' this year are small, 2 motors might not be
that big of a deal if you avoid pushing matches.
edit -- I'd use a 12:34 sprocket ratio instead of a 12:28. Single-speed geared for ~10 ft/s.