That's strange!!

Plug 3 and plug 4 are connected to seperate coils! Plugs 1 and 4 are fired by the same coil so if 4 has a very weak spark, then 1 should be equally as weak! As with 2 and 3.

If you had said that it's not firing on 1 and 4 together, or 2 and 3 together, then it would be more logical that the coil pack or the wiring to trigger it was U/S.