Make sure that you have activated correct virtual environment in which you have installed all your python dependencies before running python pgAdmin4.py
For reasons unrelated to pga4, I had to reinstall it in a new venv. I am now on Python 3.6.3 and Ubuntu 16.04, and I am working with pga4.2.0, which is a newer version than I had before, and which worked with Python 3.5. I am getting the import error related to flask that I have seen many other posts about, but none of the solutions work for me.