Check the manufacturer's minimum allowable compression limit; to see whether 150 psi is below that limit. Also check the compression in the other cylinders. If any cylinder pressure differs from the #4 cylinder pressure by more than 15 psi; the compression variation is out of specs. In that case, and also if the minimum allowable compression limit is more than 150 psi; the engine will have to be repaired mechanically before it can run properly.
Other possible sources of a cylinder misfire would be vacuum leaks; from either bad intake gaskets, cracked or warped manifolds, damaged or misrouted vacuum hoses, a leaking PCV valve connection, an EGR valve which does not close properly, or loose manifold, EGR valve or throttle body mounting bolts.
A clogged fuel filter, a clogged or defective fuel injector, or a defective fuel pressure regulator could also cause a cylinder misfire.
This motor is designed to use either double Platinum or Iridium spark plugs; with the gap set to .043". If the wrong type or wrong heat range plug is used; or if the gap is not set properly; the cylinder will miss.