Does it misfire all the timeor only when cold? There are number of things that could cause a misfire.
Fouled, cracked or incorrectly gapped spark plugs
Powertrain Control Module (PCM) calibration
Plugged EGR orifice(s) in each primary port of the lower intake manifold
If you have a 3.8L engine remove the vacuum hose to the EGR valve and power brake it. If your van stops misfiring then you have plugged up intake ports causing too much EGR to cylinder #4. You would need to remove the upper intake and clean out all the little holes of carbon. Now if it is not running rough or you dont feel it misfire you may want to call Ford and ask if your vehicles PCM can be reprogrammed " updated"