Running on three cylinders

My car seems to running on three cylinders. Replaced coil pack, plugs, leads and cylinder four injector. I know it is cylinder four since when I take the plug leads off one by one cylinder four makes no difference. Could it be a blown head gasket or blocked throttle body? I notice that when the car is sat their idling the tick over slightly increases and decreases by its self.
Car does not have an ECU so cannot diagnose electronically.

Any help much appreciated.

Regards, Mike.