Previous owner could of cracked a plug installing them.

I believe #4 plug is the hardest to get too, he could of cracked that one or any other, giving you a misfire.

If its not a plug/wire. Swap the coil pack, if the misfire follows, then your Coil pack is bad.

If that doesnt work, have your ICM tested. Also a clogged fuel filter could be part of the problem too.