Can I connect a water heater to an existing dual-pole breaker?
With having 4500W elements, the 2-pole 20A breaker will not handle the load.  Based on 4500W at 240V the heater would be drawing close to 19 amps and will likely trip.  You could swap the elements for 3800W which would draw closer to 16 amps.  Your water will just take a little longer to heat up to temperature.