Series ballasts use two wires each for individual and common connections to the pins at each end of a fluorescent tube. Series wiring uses a non-shunted lampholder, with each side of the lampholder isolated from the other side. With a two lamp series ballast, if one lamp fails the other lamp connected to that ballast will not light.
4 lamp fixtures containing series ballasts will usually use two 2 lamp series ballasts (see diagram below). 4 lamp fixtures containing HO (high output) lamps use one 4 lamp HO ballast (diagram not shown).