Hi Mark,
You have to use same python version while building and running pgAdmin4 application.
If you want to compile pgAdmin4 runtime application with Python 3.4 then export "PYTHON_CONFIG" variable so that application will use python 3.4 while compiling.
Hope you have used python 3.4 while creating virtual environment.
e.g. export PYTHON_CONFIG=/home/markm/.virtualenvs/pgadmin4/bin/python-config
After exporting above variable - while executing "qmake" message should be displayed like "Python3 detected.".
Let us know if you face any issue.
Thanks