If you want just a solid 4 ch to do the functions that come with the tank you can't go wrong with the futaba skysport 4YF 2.4. The only mod I do to this TX is wire in a momentary button on the cannon channel potentiometer so you don't have to do the trim up/stick up movement to fire the main gun. If you want more available channels for novelty functions their are a couple options, but I'm not familiar with those radios. Some others will chime in I'm sure.
http://www.ebay.com/itm/Futaba-4YF-2...IAAOSwEzxYPnuC
http://www.ebay.com/itm/Black-10Pcs-...YAAOSw37tV~iaS