422.13 Storage-Type Water Heaters.
A branch circuit supplying a fixed storage-type water heater that has a capacity of 450 L (120 gal) or less shall have a rating not less than 125 percent of the nameplate rating of the water heater.
4500 / 240 = 18.75 x 1.25 = 23.4 amps.
This puts you into a 10 awg, you could use a 25 amp breaker but most just use a 30 amp breaker.
