Hi,
AFAIR we intentionally disabled it. The reason for this is that due to how the reload mechanism works there are some side-effects which was causing executing certain code twice.
Check commit: git log a49a3103e298fcc437
If you want to enable reloading for development purposes then you can simply change value of use_reloader option to true, Set use_reloader = True in ../web/pgAdmin4.py +154
Restart pgAdmin4 and it should reload server whenever you save the changes in python files.