I bought a 2001 Ford F250 with a 5.4L and what I belived to be a bad cylinder. Well I decided to confirm this with a compression test, however the plug threads are gone so hopefully that is the only problem. I did try to check compression on a couple of other cyl's but couldn't get the gauge to screw in, except for #4. Tried to look down the hole to see the difference and it looks like a fully threaded insert in #4. Does this make sense?

 

Looks like at one time it blew a plug out of the head and wasn't repaired properly.
Timesert makes a good repair kit.