@PGregory obviously the official response is that the HS210 isn't guaranteed to work in anything more than a 3-way setup. However, the fact remains that a 4-way, 5-way, 6-way etc. setup is just a 3-way setup with one or more 4-way switches added to the traveler wires between the two 3-way switches. So there's really no wiring-related difference. But, as another forum user first noted, the longer traveler wires create a capacitance effect that throws off the HS210's current-sensing circuitry that it uses to determine whether or not the lights are on. This problem is compounded by the fact that most of our modern loads are capacitive (LEDs, CFLs). The solution, at least in most cases, is to add a 1MΩ resistor (I used one of these: https://www.amazon.com/dp/B08QRF4DM7 ) across the traveler terminals of the HS210. I would add it to the line-side switch if you have two HS210s in the circuit, though you only really need one. This resistor acts as a "drain" to reduce the capacitance between the traveler wires and allow the HS210's current-sensing circuitry to properly do its job.