I will need to connect with our dev team on that DataCloneError to see if we can provide a fix or better work around. In the meantime…
Could you explain the use case for this set up, or the code for the custom component as you have written it? It would be easier to advise an implementation if I understood how it will be used.
It sounds like you are using onChange within your lists subFields itself, but as I understand it I think you should be able to just use onChange on the parent List to limit the length to 4. It is a bit of a work around, but if needed you should also be able to access the properties of the subFields from within the list onChange…or could you possibly manipulate them directly within the component itself? Again any clarification on use case would be very helpful here.
As for the number field, in this instance you might be able to use a string with the appropriate regex limitations, instead of number type? Would that achieve your end goal?
Once I have a better understanding I can help with implementation.
Finally, for the communication with users there is no support on the list input type for message, unfortunately. Currently the only support for any sort of user messaging within the Visual Editor UI would be what you have available within the onChange function itself…most likely the alert() option you mentioned.