Going by my Service Manual for my 2003 Ford sedan...my plugs are good
for 100,000 miles.  I might change them around 60,000
though...maybe...have to think about it.

First coolant change is at 48,000 miles.  After that, every 30,000
miles.  Although DEALER said first change is at 30,000 miles ( that
isn't documented anywhere though ).

Right now, I'm at the 24xxx mile mark, according to the odometer.
I'll just keep running it until the right numbers show up.

Lg

Plugs don't really wear out from sitting, but you do tend to "become
one with" the heads.  I would change them for no other reason than to
stop them from seizing and making a nightmare later on.  

What kind of coolant is in there?  Old green stuff or the new long
life kind?  I like to change the green stuff every other year.  I only
have one car with the new orange stuff and I have been changing it on
the same schedule just because I don't trust anything new....  Your
antifreeze also has anticorrosion protection.  While the freeze
protection may still be good the anticorrosion protection is likely
gone.

Also consider flushing the brake fluid.  Brake fluid absorbs water
from the air and the water will cause you rust and corrosion issues
which seem to be a lot worse on the vehicles that sit a lot.  I flush
the brake fluid every other year as well.

                     Steve B.

Plugs, wires, caps and rotors need to be changed at the manufacturer's
recommended interval or at least inspected and tested upon the
manifestation of any symptoms of a 'miss'.  As plugs age the gap
(resistance) increases causing the coil(s) to load heavy and fail
prematurely. If enough resistance either through wires or plug gap or
if grounding develops you'll notice a secondary ignition miss usually
diagnosed by driving up a hill with the converter locked at a steady
load. You'll feel a shudder or miss which may not be noticeable any
other time.

Much discussion in the industry about extended life coolant but best to
err on the side of safety by change coolant every two years (IMHO). As
Steve B. stated antifreeze eventually looses it's anti corrosive
qualities sometimes allowing an electrolytic action to begin in the
engine often sacrificing the freeze (core) plugs or the water pump
impeller.  Depleted coolant may not allow your engine to freeze up but
it does other bad things.

Brake fluid is hygroscopic and must be tested for water absorption. Too
low of a boiling point will not only endanger your stopping ability but
will play hell with some ABS systems. Brake fluid can look great but be
moisture laden. Get it tested.

I would suggest fuel system cleaning every 24,000 miles. Use a system
which both cleans the rail and injectors restoring the correct spray
pattern. I prefer a two line system in which your fuel pump is disabled
and your engine runs via the cleaning machine which can de-carbonize as
well. Avoid the 'pour into the gas tank additives' as they are not
usually strong enough to restore clogged injectors. Instead use a 'top
tier' gasoline if available in your area switching between two or three
brands every five thousand miles as the detergent additive in gas is
paramount in maintaining your fuel injectors and intake valves. Avoid
cheap no name gas. The penny or two you save in immediate fuel costs
will more than likely cost you two or three fold in the long run.
Best regards

Plugs are probably not a high priority.  I would change these fluids:
Brake, cooling system, transmission, oil, power steering.  What's the
interval for cam and accessory belt changes.  If they haven't been
changed I would do it now.

I think of spark plugs as degrading as a function of mileage and engine
condition (too rich?  too lean? burning oil? detonation?...) rather
than time.  Why not pull one and see.  The repair manual have charts of
what you can infer from their appearance.  I'm guessing that at 20k
they're alright, on a modern car with high-energy ignition and
closed-loop engine controls.  Bu seeing is believing.

The contacts betwixt cap and rotor (dunno if this applies to you, but
it remains in use as a convenient physical format for distributing high
voltage on a lot of newish engines) might be getting a little crunchy
by now.  This is an easy visual inspection.

Plug WIRES can degrade from age as well as mileage (indirectly, because
of engine heat) but at four and a half years (assuming they were
changed at the same time as the plugs) they're probably okay unless you
live in hard desert.

How old are your hoses (including little ones in obscure places on some
models), thermostat, and radiator cap?

Just some thoughts, worth what you paid if your connect time is cheap,
--Joe