I've recently been studying some Manifold Theory and got very interested in their topological as well as geometric properties. From my understanding of the current literature, most the big and important open problems involving $3$-manifolds have been resolved in recent times following Perelman's remarkable proof of the Poincare Conjecture. In fact, many of the open problems listed in this MO post have now been resolved.
However, $4$-manifold theory still seems to be a promising area of research (including topics from mathematical physics). So I was hoping people here could briefly explain some of the main open questions and programs that are motivating current research on "$4$-manifolds and/or their interplay with Topology."
Additionally, I wanted to know if the Smooth $4$-dimensional Poincare conjecture is still considered open. I could not find anything convincing on the internet!