The section of the camshaft over piston #4 slides freely out of the rear bearing, so I suppose it's possible that the valves were both closed and the piston still will be OK.

I didn't try to turn the camshaft by hand (er, wrench). I suppose the timing chain could be broken lower down in the engine, but I would think that the momentum of the camshaft spinning as the engine died would have thrown it off the camshaft sprocket.

If the timing chain is still intact, then pistons #1, #2, #3 should also still be intact, since the cams (even the broken half-cam) should have kept them in sync with the crank & pistons.

I guess best-case-scenario would be that all my pistons are still OK if the chain is intact and the valves for #4 were both closed. But, that doesn't explain why it won't turn over with the starter. :sigh: maybe I'm being too optimistic.