Which elements should I buy for my hot water heater?
I agree with both above answers but don't jump into throwing parts at it, get an ohm meter and measure the resistance of the elements with the breaker off the conductor should be open to ground and have a low resistance end to end , sometimes both elements are good and the controllers are bad or 1 controller bad possibly just 1 bad element,  you might save quite a bit by checking things out. Longer elements at the same 4500 w is lower watt density if they will fit go long the last longer. Also don't forget about your anode it probably needs replacing.