I have some hall lights with 4 switches. Is it possible to add a motion detector switch as one of the switches and have it turn the power on and off while still using the other three switches for power on and off?

Can work in a 3 way switch location, If it needs to be located at a 4 way location, travelers need to be in and back out so it is now a three way switch location.

You're just ASKING for a headache, aren't you? If you can even find a 4 way motion detector it'll cost you plenty.

That's why I mentioned locaring at a 3 way location, or relocate a 3 way to desired motion location.

You can do it if you take the signal from the motion and use it to activate a relay that has double pole double throw. Connect the wireing in between 4 ways as another 4 way
There are small relays that could fit in one of the 4 way switch boxes.

You can do it like evolkmar mentions, and use the DPDT relay, cross the corners as you would a DPDT switch, as in polarity reversal. Relay would have 4 wires coming off relay which would basically swap the pair of wires(travellers). If it sounds complicated. Stick with the 3 way unless the location is at a 4 way location.
Where do you want the motion?

WOW! We forgot 1 big problem, when you turn on the lights by the switches, the motion will turn light off. So forget relay just wire up the same hot of the system to a motion and to the last wire on the switching (on the last 3 way switch) or to one of the lights directly.

A "four-way" switch circuit includes two "three-way" (SPDT) switches. The remaining switches are "four way" switches (DPDT reversing switches). A Lutron Maestro series 3-way "occupancy sensor" switch, for example, can be placed in either of the locations that has the "three way" switches, but not in the other location(s). When you do this, you must follow the instructions from the occupancy sensor to modify the connections to the three-way switch which is not being replaced, as well as the one that is being replaced.

You can tell which switch is 3- versus 4-way by counting the number of wires (and usually screws). The 3-way switches have three connections and the 4-way have four.

Joe,

You are aware that you are responding to a question that is 5 years old, aren't you?

Yes. Nothing wrong with that. A Google search led me here, so it is still relevant.

But the bad news is I was wrong. You cannot wire the Lutron Maestro as I described in a four-way application, nor the similar Leviton. Sorry!

Since this has been brought back to life, a good solution is the Insteon 2477D, or the 1000 Watt 2477 DH, 1 will be installed at each location.
They just need a hot and neutral, no travellers, may need to reidentify travellers to become a hot and neutral for some of the switches, these are Dual Band(RF and Powerline). One of the these will use the load wire and connect to the lights switchleg. These solve many problems such as Stairs with only a switch at one end, and no way to get travellers between the 2. Can also get the Insteon Hub for remote control.
Buy at Smarthome - Sensors - Occupancy & Motion Sensors