[TeenerTim]

How can you run a leak down test on a cylinder with no compression? Likewise, if you have no compression, no amount of re-torquing will solve that problem. I also think it is safe to assume the head came back from the shop with no problems. That leaves us with improper torquing causing the head to warp, bad or damaged head gasket, or some other problem with installation. I tend to doubt these are the problems.

Stand back and look at the situation, you had no compression in #4 when you started. After doing all of the head work you still have no compression in #4.

Conclusions:
A) the head was not the problem (never true in a 944)
B) the bent valve caused additional problems such as a crack in the combustion chamber that wasn't discovered (I would think a crack here would have been seen)
C) the piston was cracked by the valve

Anyway the head has to come off and you need to examine every surface for any defect.