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

[30psi]

Get an old spark plug, hack it and drill it out so you can get something resembling a hose on it, or use an old compression tester and hack it.

You then only need to apply actually low pressure to identify a leak. I used an old pressurized gardening water can thing for this.

Then use a foot long petrol pipe, shove it in the inlet manifold, oil filler, or exhaust/turbo to see where you can hear a hiss.

If it does turn out to be rings, or a damaged piston (eg detonation) then I wouldn't bother attempting a repair. Be easier to find another engine