215.4(A) limits super neutrals to 3 sets of 3-wire feeders, or 2 sets of 4-wire or 5-wire feeders. 4x 120V circuits would be 2 sets and allowable, would it not?
The size of the neutral would be determined by the maximum current. If this circuit were balanced- EG, 2x one phase and 2x the other, the neutral must be rated for 40A and #8 THHN is acceptable... #10 is limited to 30A by that pesky 240.4(D). If all 4 of those were off 1-phase; well, #1, 215.4(A) would not allow it, and also, it would have to be rated for 80A, and would require #4 THHN.
