It isn't at all clear how many wires you are talking about and what they run to.

Some principles: Neutral wires related to circuit loads on different breakers must be kept separated from each other. Each load on a given circuit breaker must have a neutral path back to the main panel that connects outside the main panel only with loads on that breaker. The neutral must never be switched.

For the light circuit, there may be a true neutral carried through the switch boxes to the light, or there might not be a neutral or continuously hot but rather a switched leg and travelers, in which case the wire colors may be confusing, and dependent on whether it is individual wires in conduit, 3-wire + ground NM (Romex), or two 2-wire NM (hot, neutral, and two travelers one of which is white, with hot and neutral feeding other loads as well). It depends on whether the power source is at the light or at the other end.

Here is a link that might be useful: previous thread

Neutrals should stay with the circuit for which they serve. Except for some fancier dimmers/home auto stuff, switches don't get neutrals, they typically bypass the switch and head for the load.

You'll have to explain just where these neutrals are coming from and where they are going to.

As has been stated before, keep the neutrals for each switch circuit (3way or 4way) separate from any other switch circuits, regardless of whether they are protected by the same breaker.
All the grounding conductors (bare wires) in the same box should be connected together.

The situation in question is a light/fan combo where each can be controlled from three locations. The power (12/2) enters at the first set of three-way switches, two 12/3 wires run from there to the four-way switches, then more 12/3 to another set of three-ways, then 12/3 to the light/fan combo.

On each of the sets of three-way switches, all the neutrals need to be tied together because at the power source there is only one neutral (white) coming from the panel...and at the fixture there is only one nutral (white) going to the light/fan combo fixture.

My question is concerning the four-way switches in the middle of the circuit. My understanding is that the neutrals (whites) for each switch should be separated from each other (just as if the two switches were not in the same box), but ALL the grounds should be tied together.

A builder told me that all the neutrals should be connected together. I don't think he is correct, but I wanted to ask here to be sure.

Thank you!
I thought I was right about this. My father used to say that "even a blind squirrel will find an acorn every once in a while".

Code requires that all current-carrying conductors be contained within the same raceway/cable. But the intent of the requirement is that the same current flowing towards the load also returns from the load within the same raceway/cable so that the opposing magnetic fields surrounding each conductor will cancel each other out.

In this case it doesn't really matter whether or not the neutrals are connected at the 4-way switches. The problem here is that the builder has chosen to wire the circuit in such a way that the neutral current is dividing onto parallel conductors in parallel cables between the 2 sets of 3-way switches.

With only one set of switches in the "on" position, one load is being supplied through one set of cables, and the returning neutral current is evenly-dividing between the 2 sets of switch cables. With both switches on, each cable will carry the hot current pulled by its supplied load, and they are not likely to be the same amount of current, while the returning neutral current will evenly divide between the 2 parallel neutrals. Only if both loads draw the exact same amount of current will the magnetic fields cancel each other out in each cable.

Because the loads requires 2 switched hots and 1 common neutral, the builder should either have:

taken power to the loads first, dropping the neutral off there and run separate cables at that point to and from the sets of switches.

or:

taken power to the first set of 3-ways and then suppled the loads from that point through a single 12-3 cable containing the common neutral and both switched hots.

I agree with the two option above.
The problem you have now is that you are not permitted by code parallel conductors under #6. If you were to use the second white wire in the second cable as a neutral you have parallel conductors from the first switch to the last switch.
The only option I see is to leave the one white disconnected at all locations. However I'm not sure that doesn't violate some other code about all conductors being together in the same cable or raceway.

Is the rule that you can't parallel small wires, or is it that you can parallel them but can't count on any increase in ampacity from doing so?

One big (if not only) reason for the restrictions on paralleling conductors is that it is difficult to assure the current will share between them. If there is a difference in length, or one connection is slightly higher resistance than the others the current will be mostly in one conductor and could overheat it. Thus two #12 wires in parallel are still only good for 20 amps.

"Is the rule that you can't parallel small wires, or is it that you can parallel them but can't count on any increase in ampacity from doing so? "

I don't know the code wording. I think you could be correct. You definitely can't parallel two #14 and use them for 30 amps. It would make sense that in this case they could be paralleled. They would never be carrying over 15 amps and each wire individually can handle that.

samneric,

I am having trouble seeing how your second option, "taken power to the first set of 3-ways and then suppled the loads from that point through a single 12-3 cable containing the common neutral and both switched hots" will work.

Let's say the light circuit uses the black and red in this single 12-3 cable for the travelers in its circuit. What wires are left in the single 12-3 cable for the travelers for the fan's circuit?

Sorry, it was an incomplete description with two unmentioned cables to the other 4/3 ways assumed on my part.

The first 3-way box would have 4 cables entering it:
one 12-2 supplying power, one 12-3 taking the common neutral and both switched hots to the loads, and two 12-3's to the other 4/3-ways to be used as travelers and returns back to the first 3-way box.