EDIT Teve exatamente o mesmo problema, conseguiu resolvê-lo assim:
sudo apt-get install virtualenv python-pip libpq-dev python-dev
cd
virtualenv pgadmin4
cd pgadmin4
source bin/activate
bin/pip install https://ftp.postgresql.org/pub/pgadmin/pgadmin4/v2.1/pip/pgadmin4-2.1-py2.py3-none-any.whl