Low compression on No.4--Ideas ???

[northerner]

I've been working on an S12 Silvia that I got on the cheap which had a Blown Turbo. Well it was the worst turbo I had ever seen. The turbine had completely blown up as the bearings were shot.
Having fitted another to get it running, I have found that no.4 pot has only 100Psi were the others are about 155. Having added some oil to the bore to see if it made a difference, I have found it didnt so Not the rings. I'm now thinking HG or maybe a valve problem possibly the Valve seals. It does smoke a little but this maybe from the Turbo as it has play and was a 2nd hand one. Before I remove the head anyone got any ideas ?