contact matt at www.mgreptiles.com and he will help you with all your needs. he is a great guy and everyone here will agree. also, his username here is justcage, so if you want you could probably pm him. i have a shipment of flexwatt on the way with a ranco thermostat from matt. for your 4 foot long tank i would get 2 feet of 11in flexwatt with a thermostat( which kind depends on what you are willing to spend).
that would be best for the warm side. BTW, what are the temps for your hot and cool side of the cage?