Hi all, help needed!!

Was driving around today and all of a sudden the 6 started to run like crap..just like it was on three pots. No issues with the water or oil temps no pressure in the rad system and no water in oil/oil in the water. Limped home about 20 miles and used ¼ tank fuel.
I have checked fuel to the pot that is off this is ok, got spark from coil pack as well. So completed a compression test and cylinder 4 is only producing 150 ( took a good few turns to get it to this figure, seemed to make 3 jumps to get it) and checked cylinder 2 and this was closer to 260.
It does start but takes a while to catch.

History, had a bent conrod in cylinder 4 when I bought it so replaced conrod and piston. Head was rebuilt with new guides, full set inlet valves and all valves lapped back in.
The car has covered about 9k since rebuild,cant understand it.

i have done my searching however cant find anything relevent..so any help would be great..

My plan is to pin it all up (crank and cams) then put some pressure down the spark plug to see if I get any air out the inlet or from the exhaust, to check for any valve issues..

The only smoke I have got is from the un burnt fuel that is pumping out, you can also smell it in the air.

Unclipped the wiring to each coil pack one at a time, removed pot 4 and no change to running, as I removed the others then it dropped that pot as would be expected.

Will check compression on cylinder 3 to see if they are leakinf between however cant see that being the trouble.

My father in-law is a mechanic and he cant work it out either, our feeling its not th head gasket however Still going to have to take it off..