During the installation process of Syncplify.me Server! v4.x (or greater) you will be asked the IP address and port to which the new HTTP REST API service should bind. While in most cases 0.0.0.0 (all interfaces) is a safe choice for the IP address, it is important to carefully choose a port.
In order to help you choose, we have prepared a very easy diagram. Just answer the questions on the diagram, and you’ll know which port (not) to use.
This said, in order to limit automatic probes (bots), choosing a non-standard port is probably always the safest way to go.