Dimmer Switch ?

Want to fit a Dimmer in my hall, 3 seperate light switches are already in the hall which all switch on the ceiling lights from diff. areas, only one switch is a single so Dimmer is to replace this one. Problem is it has 4 seperate connections & 4 wires going in, L1 L2 at the top & L3 L4 in the bottom, all wires are the same colour!! Dimmer has only L1 L2 & C......how do i wire this?
Many thanks Ian