I bet your using a digital volt meter right? Its called stray voltage. Its capacitively coupled causing a voltage reading on a digital meter.Electricians will have a different type meter. Are you having a problem of some sort?
I have the same issue he does - my 4 way system (3 switches, 1 light) has 48 volts across the black(hot) and white(neutral) - there is NO LOAD (no LAMP connected) just my meter. I have a 3 way circuit that measures just like I would expect it to. I have no triacs in the circuit so from the other comments about reactance in the circuit - I am not following where that would come from. Since I can see the path of the wiring, I can't buy into inductance either. Somebody have a real answer, I know that sometimes a bad neutral can cause an issue, so I am wondering if the neutrals are somehow crossed on the 4 way and the 3 ways are switching the hot. I have not pulled the switches out of the wall to verify - but was hoping someone could just say hey that is an issues with this or that - and I can go right to it with out having to really isolate each of the wires and draw out the diagram of the connections - because the way things look now - I will need to draw the diagram of how it is wired to make sure it is correct (it probably is not) I have found other simple cross up of wiring in the house so it would not surprise me.