2002 Other Chrysler Models

Noises problem
2002 Other Chrysler Models V8 Four Wheel Drive Automatic

I install a used V8 4.7 engine in a 2002 Grand Cherokee Overland. I started the engine, it started with no problem and the engine was idle with no problem. I checked for water, leak and oil leak the engine was idle all that time. Suddenly the engine started to make noise and miss fire. I remove the eight spark plugs to check for compression. Number four piston had 25 PIS pressure and the seven others had 125 PIS pressure.
I remove the right cylinder head to check to see if the valve was sticking, I also check the timing chain for noise but every thing seem to to alright. Also I check number four piston but it was alright.

Can you please advice me on what else should I check?