I have a 240 volt 20 amp receptacle in 4x4 box. There is a neutral wire not being used in the box. Can I use (share) a 120 leg of the 240 with the neutral to a 120 duplex receptacle in the same box coming off the 240 circuit breaker? There is nothing on the 240v line now, but I want to have it available just in case I want to plug my table saw in and have 120v available at the same time. This is in my man-cave and every 240v and 120v receptacle is on it's own separate circuit breaker.
Check what breaker you have first!
Amazingly enough, this is actually permitted by Code under certain (common, but not universal) circumstances!
What you are creating is a multiwire branch circuit as per 210.4. Normally, only line-to-neutral loads are allowed, but IF the breaker for this circuit is a two pole common trip unit (most two and three pole breakers are, although if the circuit is fed from the outer sections of a quadruplex breaker, you definitely can't do this as there is no way to get two common trips in that form factor), Exception 2 to 210.4(C) allows the circuit to feed both line-to-neutral and line-to-line loads (and even three phase loads for wye connected three phase MWBCs)
210.4 Multiwire Branch Circuits.
(A) General. Branch circuits recognized by this article shall be permitted as multiwire circuits. A multiwire circuit shall be permitted to be considered as multiple circuits. All conductors of a multiwire branch circuit shall originate from the same panelboard or similar distribution equipment.
(B) Disconnecting Means. Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates.
Informational Note: See 240.15(B) for information on the use of single-pole circuit breakers as the disconnecting means.
(C) Line-to-Neutral Loads. Multiwire branch circuits shall supply only line-to-neutral loads.
Exception No.1: A multiwire branch circuit that supplies only one utilization equipment.
Exception No.2: Where all ungrounded conductors of the multiwire branch circuit are opened simultaneously by the branch-circuit overcurrent device.
So, to do this would require a hot device screw with two connections. Or a pigtail in one hot. Not a good plan.
Secondly, one leg of the circuit could potentially have more load than the other if both a 120v and a 240v load were engaged. Also not a good plan.
A small sub-panel would be my first answer.