If you run web servers on each BBB, you could use "wget" or "curl" to send requests using GET or POST commands between the boards. That would be totally fine for a command rate of, say, one command per second. You don't need to use something like Apache for your "web server," you could use Python with SimpleHTTPServer, or Node.js with its http server listener, and tie code to the responses.
Another option is to use SSH. You can ssh to the other machine, and issue commands using shell commands/scripting. You can even have a local shell script do input/output through a pipe to the remote ssh process. This can probably get to several commands per second, as long as the 4G network cooperates.
Finally, if you want high-capacity remote control, you want to use a protocol that you implement using some program. If you're OK with some commands perhaps mysteriously disappearing or arriving out-of-order, as long as most commands make it through as quickly as possible, then use UDP sockets to do this. If you need all commands to make it there, in order, then use TCP sockets. You can write those programs in Python, or Node.js, or C++, or any programming language you want, really.