Once the truck is warmed up, I notice that the idle fluctuates about ~100 rpm every 4 seconds.

What would cause this?

 

id have to say the idle is adjusted wrong, or you have a vacum leak

 

I had that problem with mine and it was one of 2 things :
Aftermarket Plugs I just installed had factory .044" gap and I didn't check them , supposed to be .031" for 3VZE .
And the o-ring on the Idle Adj Screw on Throttle body was worn out and leaking and the idle would creep up when I stopped , replaced o-ring on idle screw and greased it (had to use one out of plumbing o-ring kit because dealer said they could not order it) .
Not sure which one solved the problem , I beleive both were a definate problem but after rectifying those 2 problems I had a rock solid idle rpm

 

I have stock plugs and checked the gap.

Did your idle go up and down every 4 seconds or just move up?

 

mine moves around a bit like your talking about.. ive changed plugs, rotor, cap, wires, tps, 02 and it hasnt changed in the slightest.. no vaccuum leaks i can find.. i just attribute it to being an older car and i dont worry about it

 

Thinking back it would just come up slowly until about 1100 rpm and then would stay there , I also ran the seafoam thru my intake(sucked in vacumn line) and the gas (in about 8 gallons of premium) about the same time so in reality my problem could have been a vacumn line in the throttle body clogged or even a dirty injector .

 

Lemme guess...

It goes up and down only when the AC is on and the brake is pressed right?

If so, search the forum for 'fluctuating idle' and you'll get about a zillion hits.

 

If it does it only when the A/C is on then maybe try raising the A/C Idle a little if it isn't idling high enough that might fix your problem.

 

If that isn't the issue then listen to Ovrrdrive because I have seen a thread on the forums in regard to a problem with idle fluctuation when foot is on brake and they have a mod for that in here somewhere where you can cut the wire going to the ECM from the brake system that will stop that too .

 

A/C off.

Just checked the brake pedal idea and its still does it with no foot on the brake. E-brake off, too.

 

Look hard for vacumn leaks (vacumn restrictions-Throttle Body Clean?) , making sure the controls are in working order (TPS,MAF/VAFM,Fuel Pressure and or Regulator ,and O2 sensor) , other than that maybe you have a Injector acting up possibly .
Just out of curiousity did you check the Idle Adj. Screw on the Throttle body , it has a o-ring on it and the heat kills it , if it turns without resistance (o-ring rubbing inside of housing sealing) maybe you have a leak there , thats where my big one was and didn't realize it until I replaced the o-ring and experienced the diff. .
Other than all that , maybe timing is fluctuating a little from a worn timing chain or cam timing belt/tensioner ?????
I remember reading a thread too where some of the vacumn lines got clogged but if you check all the controls for vacumn/switching then that would rule that out , also a sneaky vacumn leak is the Vacumn Switch behind intake (coolant temp switch) if you have one , they can crack and leak badly without looking bad .

 

I had a similar problem that was caused by clogged water channels not allowing water to flow to a sensor on the intake. I used an air compressor to blow out the hose lines that go in and out of the intake.

 

This is true too ( I have the V6 though) on the 4 banger I have read some threads that state that on the throttle body I presume is a engine coolant passage and it can get clogged and have adverse effects on idle/performance if not clear and able to sense coolant temp.
Several threads and people that had experienced that it was clogged and a reference to a couple of mechanics that had said they do that regularly since they had seen it many times before.