I don't think valve stems seals will cause a misfire code (it did not on my '04 545i). My bet is on the wire connector at cylinder #4 coil. These connectors can be finicky and easy to believe you have it connected properly when it is not. Ensure the connector is fully seated and latched closed properly. Warning: Looks latched closed, but if you look close you can see it is not proper if that is the case...has happened to me more than once on several of my BMW's with similar connectors.