is it possible to communicate ue4 game with a c++ program (Like hytale model maker)
There no such thing as “custom c++ software”, only C++ and you can do anything in UE4 that C++ can do… so practically anything, it more of a question if hytale model maker (which assuming will be closed source) will provide any communication features and if they can be directed to UE4 also if Hytale tool licence lat you use them with UE4. Also oyu would need ot provide proper conversion of data that those tools send to be UE4 compatible.
But if you want to make your own tool, ofcorse you can, not to mention you can build tool on top of UE4 editor (or game it self, as it seems this is what oyu want) so you dont need to start from 0. UE4 also provides live link plugin which allows to communication between UE4 to model software, but considering it using editor code you probably wont be able to use it on game runtime, or else you will rewrite so it does not use editor code
In general concept you talking about is called IPC (inter-processs communication) and there many solution that can be used, here wiki which lists some of them, most of those can be used regardless of used programming language, and some of them can be used in-between devices with incompatible hardware (most notably TCP/IP networking):