Was the same fault there before you changed the cylinder head? If so, that would point to piston problems on cylinder No4.

Are you able to do a compression test on the engine? If there's piston/ring trouble on the 4th cylinder that could cause a misfire because of poor compression.

The other thing that springs to mind is that there's something amiss with the valve clearances on cylinder No 4.

I'm not familiar with your car - does it have an overhead camshaft? Unfortunately when refitting an overhead camshaft after a rebuild, some owners have managed to bend a valve. This usually happens because the engine has been turned during disassembly/rebuild and the timing marks have not been aligned when refitting the camshaft.

This results in a valve in contact with a piston and when the camshaft is fitted and torqued down the valve becomes bent.

I think your problem may lie in either a bent valve or incorrect valve clearances.

No engines stated. there are 3.
the engine comes with 3 heads/engines.

single cam with rocker and lash. (simple and easy)
twin cam with shims. (need shim kit)
twin cam with HLA. (hydr.lash adjusters)
the real ISUZU engine names used in all Storms.
4xe1 single cam
4xe1 twin (odd how Isuzu didn't call it E2)
4xF1 , twin cam.

asking questions lacking the engine type.
impossible to help.
but some will try....

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