Century N-14 Derating

I've been getting a CEL and it reads a #4 injector open. Took it into Freightliner and the tech checked the tested the injector and the internal harness and all tested within spec. Replaced the pass thru connector. About an hour down the road truck does the same thing. Haven't made it back to the dealer to have them check it again.
Symptoms are crazy though.
If the fuel level gets low it's usually when I have the problems. Low as in say 40 gallons. Sometimes if the cruise is set it'll back itself down to 55-60 and occasionally pick up. First I was thinking the TPS harness, but now I'm not so sure. If driving with the pedal and I let off the throttle that's when it starts. Sometimes a key cycle will get things back to normal but not always. Any thoughts? Thanks in advance.

 

Would it be better to change the one or change them all out at once?

 

I ordered a replacement injector from my local shop. He wants $850 to change the one injector. Ouch!
How hard of a job is it to replace one of these?
Do I need to change the jake gasket as the guy is telling me if I replace this injector myself? The valve cover gasket is reusable right?

I pull the harness on each injector and they are dry all except the #4. Is that an indication of some leakage there from the injector?

 

Does the injector go into some type of heavy duty cycle or something when the fuel gets low?
It's odd because I only get derating when the fuel gets down to about 40-50 gallons. As long as I keep it full I have no problems, that's killing my pockets though. Only one time in the last few weeks that this has been happening did it happen while I had more fuel.
What exactly is running the overhead and should I have it done as well at the same time?