Water heater not as hot when first bought (only 4 years old) even after flushing
Like someone else said, run a hot water tap for a while and measure the temp. That's what your WH is supplying.  If at about 120 or so, the WH is doing its job.  If your shower has an automatic mixing valve, the cartridge may need to be replaced.  We had the opposite problem....the mixing valve wouldn't mix enough cold water even on the coldest setting to make the shower tolerable....it was way too hot.   Replacing the mixing cartridge solved the problem.
Bottom line, it's either the WH or a mixing valve (if you have one).