So I was buzzing around town in Wallace and almost made it to my destination when it started to run like it was almost out of fuel. It still had half a tank left, but I couldn't recall the last time I fueled up so it could be wrong... stranger things have happened. I made it to my destination and then parked it, but didn't have time to look it over. It fired up like normal when I was leaving and I made it about 4 miles of my 5 mile journey and it started acting up again.
My initial suspicion was a bad fuel filter. I had an extra so I changed it before I drove it the next time. Was about 6 miles into my 8 mile trip and it started acting all starved of fuel. If I stopped and feathered the throttle it would start to seem fine and then I take off and it would bog down again.
This is what I've observed thus far... 1) It wasn't the fuel filter. 2) It only seems to start acting up once the truck has reached operating temperature. Runs normal until it warms up, and runs even worse (after warmed up) if I try to hit the throttle hard, or am pulling any kind of hill. 3) And lastly, I did notice that the fuel injector for the #4 cylinder doesn't seem to be sealing properly. The place where it screws into the head shows evidence of some leakage, but nothing severe. It's just not sealed tightly I'm guessing, or a bad washer somewhere.
To me if acts like it has bad spark plugs. But I looked all over for them and ruled that wasn't the issue.

Thoughts?