Possibility with should/could/must/might

Question

Please share your thoughts about the following.

I am having trouble understanding why almost everyone here https://english.stackexchange.com/questions/245059/correct-modal-verb chooses "could". One guy has suggested to ask here also.

It is possible for John to shift to architecture. John (should/could/must/might) shift to architecture.

So there are four options to choose from, please share your thoughts about why the 3 of them are not suitable and why 1 of them is the best answer.

Thanks!

UPDATE
What confuses me is the preference of "could" over "might". A guy from the other forum mentioned that if it still confuses me, I should ask here.

Answer 1

It is possible for John to shift to architecture.

Let's take them one by one:

1. John should shift to architecture. This assumes that shifting to architecture is possible, but what it actually says is that shifting is recommended or desirable.

2. John could shift to architecture. This means that shifting to architecture is an option open to John which he is able to take if he wants to. That's essentially what your ‘base’ sentence means, too.

3. John must shift to architecture. This means that John has no choice: he is required to shift to architecture. Again, it assumes that shifting is possible.

4. John might shift to architecture. Like #2, this addresses possibility; but it is about the possibility that John will in fact shift to architecture. Neither #2 nor the base sentence says anything about John's intention, as this one does.