The first thought is the plant is drying out and causing the leaves to attempt to conserve moisture.
Too dry, they dry out and drop. Soggy soil causes the same thing to happen.
From this I think a pot that must drain well is important.
This plant does not like to be allowed to dry out...or be heavily soggy. It is also not one to put into a window where the hot sun can hit it directly.
Good sun....but not directly is what it prefers and with high humidity.
Keep a mister handy and use regularly.
Also, it likes to be fed monthly any quality houseplant food.

The 4" pot is definitly too small for that sized plant.
But, there is danger in going up too much, too fast.
Why not try putting it into a 6" pot...no larger and make sure it drains well. Clay might be considered.
Where roots of plants can go too far afield for nourishment, it tends to eat too much or take in too much water that the roots cant handle yet feeds the plant and trouble ensues.
You can top up the soil it is presently in with any houseplant mix of potting soil/vermiculite/perlite/sand...depending on your preference for how well the present mix drains.

Since it can react to soggy soil as well as dry, I think it is paramount to not let the plant sit in its drainage water more than a few minutes.