Questions after tear down

Compare the compression height of the # 4 piston to one of the others. Compression height is the distance from the center of the pin bore to the top of the piston. One way to do that with out a lot precision measuring equipment is, with the piston pins pressed out and the rods off, have your machine shop turn down a piston pin so it is a slip fit into the piston pin bore. Then insert the pin part way into the #4 piston and part way into one of the other pistons so the one piston pin is centering and holding both pistons. Then set them on a flat plate with the top of the piston down and use feeler gauges to measure the gap if one pistons top is away from the flat plate. I bet that you will find the #4 piston has a smaller compression height.