Water not as hot as it used to be
I have a Rheem Powervent Model # 42VP40FW gas water heater. 40 gallons, 40,000 BTU's and it is 4 years old.
Over the last couple of months the water doesn't get near as hot as it used to be. For example, my shower today I had to put it all the way to the hottest setting as did my wife (in our other shower). The water in both instances would be getting cooler relatively quickly even though it was set at the hottest setting. Both showers were in the 10 minute range or so.
Last weekend I flushed the water heater and very little sediment came out, maybe about 2 seconds worth. I used the method where you leave the gas and water on and hooked up a hose and let it run into a sink for probably 20 minutes or so.
A couple of weeks ago I turned the heat setting up 1 setting on the water heater but nothing has changed.
Is it ok to turn the heat up another level on the water heater? Should that be necessary? I'm guessing we're probably to the point where we need to hire a professional to come look at it but thought I would check for opinions here first.
Thanks in advance.