From what our NEC experts have said, it sounds as though it would be code-compliant, but I would not run travelers like this unless the wiring could be so physically arranged as to have the return conductors running adjacent throughout.
It used to be pretty common in the U.K. for lighting circuits to be wired with travelers like this or with single conductors from 3-way switch to lamp and so forth (particularly in the days before grounds were required to be run on lighting circuits).
Does anyone see any issues with using 2 lengths of 2-conductor NM cable to a 4-way switch?
No neutral being carried to the 4-way switch. Just 2 sets of travelers.
If the two runs are bundled together throughout and run from the 4-way back to the same box for splicing into an existing 3-way circuit, then the fields should cancel out just fine.
