You set out to replace some or all of your switches in a circuit controlled by three or more. You cannot remember how they were wired, and now things do not work.
(If you only have two switches in your circuit, see instead My 3-way switches don't work.
What do you do?
First, you may find it helpful to understand how 3-way and 4-way switches work to accomplish this task. See Controlling a light with two or more switches. Further, this discussion assumes that you have only disconnected the switches and have not removed wire nuts from any other connections inside the switch boxes.
Note that there are 3-way switches at each end and one or more 4-way switches in the middle. For this discussion, let's assume only one 4-way switch.
We need to answer the following questions:
You generally need two people working together answer these questions:
one with a multimeter on resistance, one tapping pairs of wires
together. Any multimeter with a resistance scale will work. You should be able to find one at Radio Shack for ten or fifteen dollars.
First, kill the circuit breaker and be sure there is no power to the circuit.
Next remove all three switches.
Post person A at the first 3-way box and person B with the meter at the four way box. B measures the resistance between various pairs of the 4 wires as A taps together various pairs of the 3 wires. When B sees the resistance alternating between zero and infinity as a pair of wires is tapped, you have identified the pair of wires going from the first 3-way box to the 4-way box.
This is easier, because the remaining pair of wires in the 4 way box leads to the second 3-way box. Place A at the 4 way box and send B with the meter to the 3 way box. Have A tap the (second) pair of wires in the box together as B measures the resistance between pairs of wires in the 3-way box, until the connecting pair is identified.
This was already answered as part of answering question 1.
We will now make measurements on the isolated 4-way switch, with the switch out of the circuit. Place one lead of the multimeter on any single screw of the switch. Measure the resistance to each of the other screws in each switch position until you find the one screw that shows infinite resistance to the first screw in both switch positions. Call this screw and the first screw the "IN" pair. Call the remaining screws the "OUT" pair.
Find the pair of screws that always show high resistance with the switch in either position. Call these the "OUT" pair. The remaining screw is the "IN" screw of the 3-way switch. Double check: the resistance from the "IN" screw to either "OUT" screw will toggle between 0 and infinity as the switch is flipped. Note: if one screw is a different color from the other two, this is probably the "IN" screw.
Now you are ready to wire the switches. Refer again to the figure on the Controlling a light with two or more switches page.
The circuit should now work.