My new project

Update time.
So today I changed the oil and filter. 10w30
Fram extra guard filter.
Then I changed the coolant. And changed the spark plugs NGK (I forgot which model)I gapped them to .035/36.
I used the opportunity to do. Compression test and these are the results cylinder 1(closest to Radiator ) cylinder 6 (closest to firewall).
1=125psi
2=122psi
3=125psi
4=80psi (retested multiple times)
5=122psi
6=103psi (retested 1 time)
Runs smooth with no smoke. Can anyone give me a good order of ways to trouble shoot cylinder 4?
I'm thinking easiest to hardest ie, check valve clearances, put alittle oil in the cylinder then retest to see if compression bumps up, I'd rather try to avoid pulling the whole head off until last resort.