Oh as someone who has been in the fandom since s2, you basically get leaks sometimes super blurry pics sometimes videos from filming site but the videos would be from big distance sometimes good quality pics if they were filming in the streets and houses near the location
In other ways sometimes fans dig around and kinda stalk accounts of the crew members to see if they post anything that is related to filming like recently Benophie fans found one of the crew members that are responsible for the set’s design and they shared her Pinterest account ( the account is public so it’s okay ) and some of her boards for s4 for the sets she is in team for (masquerade, ben apartment, the queen throne room & fran and john house)
I have to say that among all seasons so far s4 got good quality leaks from the first two weeks of filming and it was related to majorly scene of the main couple of s4 so if we might get more good quality pics in future
All I know is that so far for s4 only Luke t and yerin were the ones filming scenes so far while the rest joins them in October