A 4-way, 5-way, 6-way, or however-many-way circuit is just a 3-way circuit with one or more 4-way switches in the traveler wires between the two 3-way switches.

A single HS210 or KS230 can control a multi-way circuit. Best practice is to install the smart switch at the "line" end (where the wire connecting to the "line/load" terminal is always hot, vs. the one that goes to the light fixture). Once that's done, the lights should be switchable using any switch in the circuit, and the smart switch should reflect the correct on/off state of the lights.

But if the traveler wires are too long, which usually only occurs when you add more switches for a multi-way setup, this creates a capacitance effect between the two traveler wires, which causes problems for the current-sensing circuitry in the smart switch. So the switching action works, but the switch can't tell whether or not the lights are on. This usually results in the switch always assuming the lights are off. This problem is compounded by low-current capacitive loads like LEDs.

The fix, in many if not most cases, is to add a "drain" resistor across the traveler terminals of the smart switch. This fix was hit upon by another forum user, not me, but I have used it successfully. Testing has suggested that a 1MΩ resistor does the job. I used one of these: https://www.amazon.com/dp/B08QRF4DM7