I am converting to electric. Will 10/3 wire work? Single or double 30 amp breaker ? The heater is 4500/4500 240V

You are converting WHAT to electric? Just one heater?

For a 240v 4500 watt heater 10/2 is all that is required, and yes, a 2-pole 30A breaker.

Re-mark the white wire to a "hot" color with a Sharpie marker.