"blueman" wrote in message

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

You do not say what the warranty period is for this heater. If 10y - it may
be to early to change the anode.

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.

"As much force" is relative. I'm not sure what the neede torque is, but
it's a lot. I needed an extension on the wrench. With an extension, you
can apply continuous force and it will let go.

Any suggestions on how to loosen the nut?

Just gentle force via an extension.

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?

Of course, if you don't do it right, you can mess up the heater :-) IMO
it's worth it, if you do it at the right time. Too early and you've wasted
your effort, too late and it's too late.

RichK