Number 4 plug does show signs of it running rich.
This could've been due to the spark. You would need to remove the new plug after driving it a good number of miles under all driving conditions you normally run it through; and then inspect them all and see if only #4 plug has semi-blackened insulator and partially on its strap.
If so, and only #4 has signs of buildup, then you may consider swapping the #4 plug to another cylinder. You may also at this time, move the #4 coil to another plug separate from where you placed the plug from #4, and drive it as you did before. Inspect all of the plugs again. If the blackness moves to where you placed the coil...then it may just be that coil. If the blackened buildup always remains on the plug in the #4 cylinder...then it may be time to address the injectors by servicing or replacing, and do a leak-down test. The leak-down test may actually be best if done first.
I know this all sounds a bit much, and you may just want to replace coils and injectors, but the buildup may be due to normal wear within cylinder #4....rings or its valve guide and/or seal. These engines tend to get a bit crusty from buildup in the area around the edges of the piston face and piston rings. Not sure if this helps or only makes its future needs more daunting. Hopefully you find it is only a coil that is fading out, or an injector isn't shutting as it should.
