Could you get a readout of the exact ECU codes that you have pulled. They will be in the form of a P plus a four digit number in the range 0000 to 1999 (ie., P0*** or P1***).

May also be a stuck injector causing the misfire and loss of power.

There is a known issue with oil contamination of the plughole in cyls 3&4. You have not stated if you have the metal shim rocker cover gasket or are still using (re-using???) the old one, or silicon etc.

See the "changing plugs" sticky above if needed (sound like you have it pretty sussed though) and change all coils leads and plugs if you haven't already (if nothing else it gives you a set of spares).

Someone else with more experience than me should pipe up with help for the helicoils.