Took it to advance this morning
 

And i had them hook a computer up to the mustang and it says that cylinder 4 and 8 are misfireing, So how can i fix this now?

You need to know hy it is miss-firing. Either those cyclinders are getting fuel or spark? I would start by checking those plugs and by checking the coil packs.

switch the coil packs from 2 other cylinders and have it re-tested. If it fails on the new cylinders then it's coil packs. If not, try spark plugs and possibly those two injectors.

Follow my write up, http://www.dndgarage.net/misfire.htm

Take another look at the rubber boots on the coils; a tiny pinhole can cause this, you may see a small, chalky white area where the spark has jumped through.

Your EGR can fail also; i had this happen to my car and it only gave misfire codes on two cyclinders. But i would be more willing to guess spark or fuel related for you. Mine went froma coolant line that blow and destroyed some wiring to the egr valve that stayed open all the time.

you'll need to replace the 2 COP's (maybe more, once you disturb them) and proby should put in 8 new plugs. cleaning is a waste of time, esp injectors as FoMoCo has used deposit resistant fuel injectors since @1992