Then they should probably keep reference to g4 as light as possible, because no one will buy that otherwise.
As with G1 for G4, you mean? The movie is likely to be full of vague references about G4, but when the TV series comes out, the background from G4 will be minimal, only the background lore will continue — races, culture, localities, history. I would be very surprised if the book that started and ended G4 did not appear at the beginning of the movie, and if the Mane 5 did not find a statue of Mane 6 somewhere out there to prove that these had existed. Maybe when the movie ends, a new book will be brought out to symbolize the beginning of G5. They will still be in Lauren Faust's creation if G5 is a continuation of G4.
To start an era, the previous one must end in an epic and symbolic final act where the new will take over. We have not come to this stage that can be in the movie. It is intended to start the G5 by picking up the track from the G4. That may be enough if Sunny gets a certain book at the beginning of the movie. When the movie ends, G5 will begin as a separate era.