I have an upper and lower 4000 watt water heater element can I replace it with a 4500 watt or a 3500 watt one with out any major problems I have a 30 amp braker in my panel

Should be OK, 4000 watts is 16.66 amps. 4500 watts is 18.75, 3500 watts is 14.58 amps. Certainly can use 3500 watt but take longer to heat water. Check physical size against old one to make sure it will fit.