P0304 Cylinder 4 misfire / head gasket issue? on 2002 Mercury Grand Marquis

4.6L v8 with mil light on code is P0304 Cylinder 4 misfire. Coolant overflow was light brownish in color. Could this be a blown head gasket? If so , what is a ballpark estimate to have this repaired

Asked by for the 2002 Mercury Grand Marquis
Common Problems that Trigger the P0304:
Worn out Spark Plugs Ignition Wires, Coil(s), Distributor Cap and Rotor (when applicable)
Incorrect Ignition Timing
Vacuum Leak(s)
Low or Weak Fuel Pressure
Improperly functioning EGR System
Defective Mass Air Flow Sensor
Defective Crankshaft and/or Camshaft Sensor
Defective Throttle Position Sensor
Mechanical Engine Problems i.e Low Compression, Leaking Head Gasket(s) or Valve Problems
are you looking to do both gaskets or only one side?
I Have P0307 cyl 7 misfire, runs rough no smoke coolant clean but how do I find out which of these problems it really is without being at the mercy of the repair shop to be honest.
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