Those four things mentioned in the “Acinteyya Sutta (AN 4.77)” are “imponderables.’
They CANNOT BE grasped FULLY by anyone but a Buddha.
– That means we can understand them to some extent, but not in full detail. So, the Buddha said not to try to understand them in great detail, for one could spend one’s whole life and not be satisfied.
– Furthermore, that would take time away from pursuing the most important objective of attaining Nibbana.
Regarding jhana: Just like the other three subjects, one will never be able to learn everything about jhana, including iddhi powers that can be cultivated.
– For example, Ven. Moggallana was only second to the Buddha in iddhi powers. But once he decided to “explore” the vast universe and got lost. Buddha had to come to his “rescue.”