FreeRTOS+TCP v4.x is quite new, so there may very well be some bugs. The primary new features are support for IPv6 and multiple interfaces.
There is one known issue related to port number reuse on multiple interfaces. If you are only using a single interface at a time, this should not affect you.
If you do discover a bug and need to revert back to 3.x, porting the driver from 4.x to 3.x should be relatively straight forward.