How to construct a 4 Inputs OR Gate?

I don't know what type of motor control relays that you are using, decades ago with the ones we had, AUX switches could be screwed to the relay. One could place a switch on each relay, and series them to activate your final function.

You just run the control voltage through auxiliary contacts for each MPCB and use a manual override momentary circuit for start up....

I agree.Keep it simple.

That would be a logical "and"; in the event of any <...MPCB...> going open-circuit, all the others do too. All four <...MPCB...> units would need to engage simultaneously for it to work, so some form of start-up timer needs to be in place, as indicated above.

Anything can be achieved by appropriate design. If in doubt, consult an Electrical Designer.

The MPCB can be purchased with auxiliary contacts. You can use NO configuration so when the MPCB is closed the contact is closed. Wire the 4 contacts in series to either a control relay or to the contactor(s)/starter(s) you are using to control the motors. Any of the breakers trip then everything shuts down.

In addition to that, consideration has to be given to starting the motors, from the perspective of a non-simultaneous start, which might be necessary not to overload the supply.

That's why detailed design is advocated. This isn't something that can be just lashed together or bought off the shelf.

Can you just use a single MPCB to drive all four motors in parallel?

In such circumstances, how can one breaker protect against one stalled motor among the other three (rhetorical question - NNTR)?

Detailed design considerations are necessary.

Consider having one motor that drives a mechanism that feeds all four mechanical loads simultaneously, instead.

Put them in parallel.

This is a simple four input OR gate

But we need a lot more information.