Hello,
For designing motors into any system I first go to JVN's Design calculator. Downloadable on the white paper section.
Under 1 speed drive I inputted then numbers and the result:
http://prntscr.com/2nk54w
Firstly notice your speed is 24ft/sec which is beyond anything you could ever need, most teams in the single speed area gear for about 14-15fps. Then notice your amp draw at over 200 per motor under load..... remember the jaq is rated for 40 continuous.
Now if you go down to say a 11:1 ratio, which i would do, you will loose speed (but still have more then enough) but gain toque which will give you more pushing power and also with a higher reduction you will use less amps due to the fact your motors are working less.
Now you can do this with a chain reduction, as stated above instead of buying 4 new gearboxes.
Best of luck.