Number 4 plug fires intermittently

I have a 1979 kz650b3 and the number 4 plug fires intermittently when tested by grounding it to the block and spinning over the engine. The other three plugs all fire normally, including number 1. It does not seem to be the plug as that plugs fires normally when I put it into any other plug wire and test it. I don’t think it can be the coil as number 1 fires normally. That seems to only leave the number 4 plug wire and plug wire cap. These plug wires cannot be removed from the coil as they are permanently attached at the factory. I did remove the screw-on cap from number 4 and trim a    1/4 “ off the end and screw it back in, but that did not change the result. I got the bike in February and it did not have coils, so these are new and have about 800 miles on them. All four plugs have been firing normally until this last check and all 4 plugs only have 100 miles on them. The bike actually runs quite strong at higher rpm and only seems to misbehave at lower rpm. The bike is now running poorly at start up and requires a few miles down the road to work out of it, even on a 90 degree day.
Joe

Can you see and cracks or burns on the plug wire. Try running the bike at night and look for arching off the plug wire. Z1 Enterprise sells a plug with splice. I have never used them. 

www.z1enterprises.com/ngk-plug-wire-splicer.html

The wires will come out of the coil.  The coil has a recess with a spike on the bottom that the wire presses into.  Get a pin and push it between the wire and coil, then squirt some WD-40 alongside.  Then move the pin around the wire and do it again, eventually you will get some WD-40 all around the wire, which will make it slide out of the coil without too much effort.  Be careful to not damage the spike post at the bottom of the recess.  Once the wire is out, you can shove a new one in place.  The stock wires and 7mm copper core.  

Ed,
thanks very much for your technique for replacing these wires. I will definitely try it and let you know the results.
Joe

Ed,
I took the plugs wires out of the coil using your advice and they came out easily. These are screwed in in the same manner that the plug caps are screwed into the other end. All I had to do was trim the cable ends and reinstall the existing wires taking care to get them on as straight and tight as I could. Problem solved!!! 
Thanks again for your invaluable knowledge of these old Kz650’s.
Joe

Hum.  Interesting to learn your coils had a screw like feature down in the passage.  That will be more secure than just a spike. 

FWIW, Amazon sells bulk plug wire for cheap