F1AT report that Ferrari has pushed on the combustion of the fourth ICE in an attempt to compensate for the 0.9 oil and add a few more BHPs than the ICE 3. They say the new ICE wasn't originally designed with the 0.9 oil rule in mind, so they had two alternatives:
put a new ICE 3 in the car and lose some performance because of the 0.9 (according to them the gap of ICE 3 to Mercedes was 20-25bhp) so that the gap to Mercedes will open to 30bhp
or accept the risk to debut the ICE 4 (+new cooling) that is pushed very hard on the combustion side, to gain the power lost because of the 0.9 oil rule and another 10-15bhp (10 according to them 15 IIRC according to motorsport).
The problems on the spark plugs would happen because of the very advanced combustion of the ICE.