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.