Is 3500watt element going to heat a little let than a 4500 watt element because my element in my water heater is 3800 watts .I want to use a 4500w element .

First lets determine the voltage to the heater. 240VAC? 30 AMP breaker? Yes, 3500 will heat slower than a 4500. Where does the 3500 come in when you have a 3800? One element or two? Depending on the supply voltage to the heater and the wire size, you may be able to swap out.

Yes the 3500 watt element will heat slower but will heat to then setting. You may increase to 4500 if you wish but in my book it would be a waste of time and money. Good luck, Tom