I think #4 just isn't contributing. I think spark must be OK. That leaves fuel and compression. I'm yanking the carbs off in case I missed something on that one. It was plugged up. I worry that it might be compression, bent valve or something. If someone over-rev'd the engine, could happen. Anyway, in this picture, the #4 pipe is lower left. hmm, lower right? The picture is being rotated left 90 degrees.