Ford Focus coil pack and ht leads ohms

hi i was just wondering wat the ohm reading should be to check the ht leads and coil pack on my car. i am nearly sure that the coil pack is nackered but just want to make sure its gone be4 i replace it. the car is only firing on 3 cylinders and i've already replaced the spark plugs and swaped spark plugs from cylinder to cylinder to make shur that one of the new plugs wernt faults but there ok. when replacing plugs i noticed that cylinder 4 spark plug was very black compared to the rest of the plugs. i have also removed ht leads 1 by 1 to c the difference in the engine performance when removed. and when cylinder 4 ht lead is removed it makes no difference to engine. any replys would be greatly appreciated