Cylinder #4 not firing...causes?

I found that when I step on the gas the motor is weak and makes a lawn mower noise. I pulled each wire out to see if it gets worse or stays the same to determine which one is bad and found out it was #4. I tried changing all 4 spark plugs and wires but the results are still the same. It has done this before but a spark plug replacement solved it but not this time. What are the possible causes? Bad distributor maybe?

did you check the contacts in that distributor cap?

Also make sure to check the contacts on the rotor inside the cap if there dirty some sand paper will clean them up there also cheap to replace. Have you done the spark test from the wire to see that its actually not sparking. you can pull all the wires and stick a screw driver in the wire to test and hold near a ground like the block or frame and have someone crank it it will arc to the ground if its good. Have you checked coppression? Or fuel to the cyl?

Try a new cap, if that doesn't fix it then you may have valve problems.