clio running on two cylinders?

my clio 1.6 1999 runs on two cylinders when it reaches normal running temp, i think i may know what the problem is but i dont know the name of the part to order a replacement. the leads run from the spark plugs to two small black box's, one box is for number 1 and 4 cylinders and the other is for 2 and 3 cylinders as you look at them. Can anyone tell me the name of these parts? thanks for any help.