I actually designed a four speed design transmission that my team (3250) used this year. We used a 3 stage 2 speed vex gearbox to start, and added a custom gearbox to overdrive or underdrive the gear reduction. We used carbon fiber and Kevlar composite panels to house the whole drive train, which was designed to drop out of the bottom of the frame with just a few bolts. The gear reductions used are 5:1, 10:1, 20:1, and 40:1 and we used pneumatic ball shifters to shift between gears.
Here's a few pictures:
http://http://www.chiefdelphi.com/media/photos/40581
http://http://www.chiefdelphi.com/media/photos/40582
http://http://www.chiefdelphi.com/media/photos/40579
http://http://www.chiefdelphi.com/media/photos/40580