PO303 and 304 codes

Ran a code check and got a PO303 and 304 cylinder misfire codes. Can anyone recommend any possible causes and/or solutions for this problem?
It would be much appreciated. Thanks.

Plugs, wires, coil pack first and easiest things I would check out first.

I have already changed the plugs,wires,and coil. Could valve float be a possibility?