running rough

can anyone help me with this problem? i got a t- reg fiesta 1.25 zetec s.
the problem is that it starts ok and then running rough. i checked the spark at spark plugs and found no spark at cylinder 1 and 4. cylinder 2 and 3 ok. i have tried changing the coil pack and no change. Could it be a timing problem or something else????
any ideas thanks
mark