The game does not end after the finale of act three. The add-ons include their own selection of main, side, miscellaneous and radiant quests which can be completed at almost any point during or after the main storyline, and which can even interact with the main quests in some ways.
These radiant quests use randomized characters, locations and other elements and often can be repeated infinitely. The game also includes a 'radiant quest system'. In addition to the usual main quest line, side and miscellaneous quests have been added.
Fallout 4 features an expanded quest system, compared to that of previous Fallout games.
