1997 Chevy Astro constant miss on #4 cylinder !!

Engine Performance problem
1997 Chevy Astro 6 cyl Two Wheel Drive Automatic

this van has an irritating miss on the #4 cylinder!!!
I have good fuel pressure & supply & no vacuum leaks,
I have switched the # 2&4 spark plugs & wires positions still the same.I have purged the injectors & intake manifold !!! it gets better but the miss still came back!!. then i replaced the #4 injector. still missing on the same cylinder.i have ok compression for a veh that has 500,000kms (315,000miles)

plz if anyone has came across this or a similar problem,you input will be greatly appreciated

THANK YOU FOR YOUR TIME !!!!!!

I found a possible cause for the miss on the #4 cylinder !!!
it can be a sticking lifter that keeps the valve slightly open or a valve that is tuliped !!!!! I found this out by slowly loosening the intaike rocker with no change. but when i started to loosen the exhaust rocker the miss started to go away!!! I had turned the rocker bolt 4 1/2 turn till the miss was gone !!!! I hope this helps any one else out there that may have this problem !!!!!!!
If there is any questions on this problem plz ask & i will replay as soon as possible..