I'd expect it to break down in pairs, but.........
One cylinder down and an ignition issue would make me strongly suspect either a lead or a plug. Try changing the plugs over (one may have given up or fouled) and then the leads, so that the coil for 1
and 4 now feeds 2 and 3.
If that doesn't get you any closer, I'd look at the wiring to the coils and/or I think the coils are common to a shedload of FIATs of the same epoch -- you could pick up some cheap at the scrappy.