We have an approximately 4 year old gas-fired 50 gallon hot water
heater.

Based on recommendations I saw on one of the TV howto shows, I tried
to unscrew and examine the sacrificial anode for corrosion.

However, I could not get the large nut on top of the water heater to
budge. I tried liberally applying liquid wrench and using as much
force as I dared so as not to damage the gas connection.

Any suggestions on how to loosen the nut?
Also, are such inspections really worthwhile in terms of trying to
extend the life of the water heater or am I best off leaving it all
alone?

Thanks