4 Pole MCB to a single phase load

You always start with 1 for L1 live wire in, and the second is L2 and so on....

Just remove the handle tie bar...

...or you could check the load on each leg and use that circuit to balance the load if needed...

??? Why would you want to allow N to turn off and leave L_x on? If it is going to trip N it seems that it must trip L_x at the same time. I would do as someone said, check the phase loads and put it on the lowest load.

How is N going to trip, it's just a switch...it can be bypassed anyway....I don't know why you would want a neutral switched in a 120v circuit, but the application is unknown...

The neutral is switched when there is a need to carry out isolation for safety. Because the neutral is a current-carrying conductor, it is considered live, and under some circumstances (loose or missing neutral ink upstream is one example) it will carry a voltage. And voltages hurt.

There is also a need in transfer switching between sources like line and generator. Which is an isolation function as well.

In a single phase application I don't think it is necessary....Yes the original application may call for the neutral to be switched, but that would be a 3Ø circuit with floating ground...

IMHO, I believe #2 is better. A 4P breaker is designed for a 3-phase load. In #2, the breaker sees the same current in all three phases. If only one phase carries current as in #1, the tripping circuitry may not work reliably.

Check your local Electrical Standards

https://www.quora.com/How-do-I-use-3-pole-MCCB-as-a-single-phase

https://lselectricamerica.zohodesk.com/portal/en/kb/articles/can-i-use-a-3-pole-mccb-on-a-single-phase-system

I guess it depends on if you have a common trip breaker or not....

https://diy.stackexchange.com/questions/201543/what-is-the-difference-between-a-handle-tie-and-common-trip-in-circuit-break

Just follow the wiring regulations applicable to the country of installation; in the UK it is British Standard 7671, where switching the neutral has special considerations depending on application.

Most people would be less confused if you just used 2 of the 4 poles. It might appear to some that you are increasing the interrupting rating of the breaker by series connecting the contacts, and send someone off on a wild goose chase when they come across this in the future. Leaving the unused poles empty confirms that you are perhaps a Yankee, for future service. It is unlikely that on this sort of unsophisticated circuit breaker that any 3 phase balance sensing would affect the thermo-magnetic trip characteristic for a single pole load.

Depends:

Four pole usually used for three phase, the fourth contact is used to 'hold' coil in 'on' position.

If contactor has also thermal overload, to use for single phase load L1 is looped back into L2; L3 is neutral; L4 not required unless used for 'breaker hold'. Thermal setting is adjusted for the load.

No thermal involved, then L1 is 'live', L2 is neutral; other two contacts are then spare. Always have full isolation of load on switch-off.

You can have either one or the third would be to have, similar to example 2, on phase and neutral two series contacts each. These protect from overcurrent and overload so most likely it will have similar current values on L and N. Moreover, it increases the arc breaking capacity on both wires.