Well, if cyl #4 is only at 80....yes there is an issue.

There are some tests you can do to see if it is from a valve, the headgasket or rings....if you want to know before going in.....but at the end of the day, sounds like the head has to come off.

 

Befor you go any futher do a valve adj. A shot in the dark but you might have a tight valve in that cyl. IF no help I would do a cyl. leak down test My WAG is A BAD VALVE