Personally I would switched to running lipos. Youll get much better performance and run times but regardless of whether you go lipo or stick with NiMH you need a proper charger. you're gonna burn that NiMH pack up fast charging it at 4A like that if you havent already. Id get a charger like
THIS. It will charge NiXX and LiPo packs either one. If thats not in your budget we can recommend cheaper chargers but that one you wont have to worry about replacing. At least not unless you get really really into the hobby.