Is there some Emacs package that defaults to listening on port 4001?

There is network process started by Emacs listening on port 4001, and I can't recollecting settings some process to listen on port 4001. Connecting to the port via browser or telnet produces JSON output if that also helps.

Not like I know but looks like atomic-chrome may listen on that port.

Yes, that is what it is. It was what I had in mind and I even checked my browser for the addon but I couldn't remember the name. Thanks.