File location (assuming you have python 3.5) is "lib/python3.5/site-packages/pgadmin4/config_local.py" relative to the pgadmin install directory. You may have to create it as the file is optional. You use it when you need to override default configuration. I like to keep pgadmin 4 configuration separate from pgadmin3 so mine looks like this
Sorry, but after an exhaustive search of the several terabytes on my machine, there is no config_local.py file. Do you have any idea where it's supposed to be located?