You'll only be able to tell if you get a compression test to see if that is the issue.
However don't lose out hope. As you have changed the coil pack and there is no errors and there is still an issue. It might be good to check that the number four cylinder is getting fuel.
Not really heard of many injector issues to be quite frank. But its worth seeing it its clogged. You need Air, fuel, compression and spark for a petrol engine to work.
So you are just checking you are getting those.
When its been running for a little while if you remove the spark plug is it covered in petrol?
If so then its getting petrol.
In which case as others said it might be valves that are no longer sealing.
If that is the case then yes. Its a head off job and replace the valves. Its not too bad of a job and far easier than a complete new engine.
So check for fuel
Check for compression
You want at least 160 psi across all cylinders. Or at least even ish numbers across the cylinders.
If you need a replacement head. I have a complete one that I can sell you. As its from an engine with a bottom end failure. So the top end is very good. 180 psi when I last checked a year ago.
However please check those above things.
Alfa 166 2.0 Twinnie - LPG and a few trick performance parts