I have a 4 year old natural gas 30 gallon Rheem water heater. Since last
week, we have had 90-110 degree water at night (which is really the only
time we use it). The tap water is coming in at 45 degrees. I live in a hard
water area.

I live in the mountains and I am wondering since it has warmed up here that
maybe our water supply is coming from some snow runoff and that could be the
culprit?

I have maxed out the thermostat. Time for a new water heater? What do I need
to check before I go spend $600+ for a new water heater installation?