I am assuming this is a 5.0?
If so, replace both cyl heads. They are reasonably priced from Ford these days due to volume of sales. The valve guides have been redesigned to stop the issue of the valves pulling into the seats. All cylinders do it, but for whatever reason cylinders 4 and 8 tend to be the first ones to show up.
I have done many of these, and all of the ones done as retail I recommend doing both heads. If one is shot the other is very close behind.
2003 GMC Sierra 3500 -- dually, 4x4, hasn't blown up yet and that's what i look for in a truck
Ford Doctor -- They come in broken and leave fixed. Ford of Canada Master Tech