I have 2004 ford ranger edge 3.0 approx 90,000 miles,that started missing on cylinder #4 it was intermittent some days would not miss, I changed the coil, (twice ) plugs,plug wires,and fuel injector on #4,never got any better was still missing, mostly at idle,now getting comp code errors missing on cylinder #4 ,#5 and # 6 and unkown miss under 1000 Rpm. 4,5,6 are all on same side of engine,I then changed the cam sync(old days was the distributor) and cam sensor.because it was squealing in cold weather start up. this stopped the squealing but not the miss on 4,5,6 .I am running out of things to try and out of money .i took it to a muffler shop to check converter but all he read was comp miss 4,5,6 said he didn't think it was converter because it didn't have the error codes that would show that, didn't do a back pressure test. only thing i know that is left would be the ECM because the wireing looks fine any suggestions would be greatly appreciated

 

Several things pop to mind immediately. Plugged EGR on the 1,2,3 side causing more flow to the 4,5,6 side.[try running with EGR disabled for a bit..] Leaky intake manifold gaskets.[spray WD-40 around the gasket joint with the engine running] Valves worn out or lifters sticking holding the valve off the seat.[compression test...]
If you have a timing light, you can use it to insure that the individual spark plugs are getting fed a nice jolt of juice. Clip it over the plug wires and watch the light as the engine is running. Look for 'missing' flashes ... they'll be hard to see, I'm sure, but you will notice a change in 'pattern'.
tom