2002 Mustang GT

I HAVE PUT IN A REBUILT 4.6l  MOTOR A YEAR AND A HALF AGO. I THEN PUT IN A REBUILT TRANSMISSION. I SHOULD HAVE A BRAND NEW CAR RIGHT? WELL THE BATTERY ICON STAYS MOSTLY ON AND THE CHECK ENGINE LIGHT COMES ON THIS TIME 6 DAYS SINCE LAST CYLINDER # 4 CHANGED. It is always cylinder #4. I have done a compression check, I just don’t know what to do. The spark plug had some wet grit on it but showed no signs of contact. So why isn’t it firing?