Q. Is a neutral conductor allowed to be switched for 3-way or 4-way switching?
A. Wiring for 3-way and 4-way switching must be done so that only the ungrounded conductors are switched [404.2(A), as shown in the Figure below. In other words, the neutral conductor must not be switched. The white insulated conductor within a cable assembly can be used for single-pole, 3-way, or 4-way switch loops if it's permanently re-identified to indicate its use as an ungrounded conductor at each location where the conductor is visible and accessible [200.7(C)(2)].
If a metal raceway or metal-clad cable contains the ungrounded conductors for switches, the wiring must be arranged to avoid heating the surrounding metal by induction. This is accomplished by installing all circuit conductors in the same raceway in accordance with 300.3(B) and 300.20(A), or ensuring that they're all within the same cable.