Single Neutral for Several Smart Switches
Keep 'em separated

Since you have two subpanels feeding this setup (one for each "side"/half), you'll need to make sure that the switches for each side have a neutral return back to the corresponding subpanel, and only the corresponding subpanel.  In other words, the two left-side switches have a neutral wire going back to the left-side subpanel, and the two right-side switches have a neutral wire going back to the right-side subpanel.

Cross-coupling the two smart-switch-neutrals in your setup violates 300.3(B)/310.10(H) and creates a neutral current loop that causes inductive heating of metal parts (like your raceways and boxes) as well as magnetic field EMI due to the loop area of the current (old-school picture tubes were susceptible to this in particular).

Neither of these neutrals needs to join with the light fixture neutrals on its side until the subpanel neutral bar, though.  The smart-switches are loads of their own (to power the "brains"), and running an extra neutral homerun in the same cable, conduit, or raceway is OK by code, as per 300.3(B) (otherwise, /2/2 NM wouldn't be Code legal, for that matter):


  (B) Conductors of the Same Circuit. All conductors of
  the same circuit and, where used, the grounded conductor
  and all equipment grounding conductors and bonding con-
  ductors shall be contained within the same raceway, auxiliary gutter, cable tray, cablebus assembly, trench, cable, or
  cord, unless otherwise permitted in accordance with
  300.3(B)(1) through (B)(4).


Load isn't a concern here, but Code is

The primary concern for multiplexed neutrals is having multiple hots on the same phase return via the same neutral, which causes the currents on that neutral to be the sum of the currents on the various hots.  In this situation, though, that isn't a concern as smart switches don't draw much power at all.

However, technically speaking, unless you punch the circuits down as such to have currents cancel (i.e. a MWBC -- such things do exist in the three phase wye world, whether they are two-phase/three-wire or three-phase/four-wire), your proposal violates 200.4(A):


  (A) Installation. Neutral conductors shall not be used for
  more than one branch circuit, for more than one multiwire
  branch circuit, or for more than one set of ungrounded
  feeder conductors unless specifically permitted elsewhere
  in this Code.