Fans were clearly very upset when Grace was killed in season 3, and her death was discussed quite a lot on messaging boards related to the show.
Whether this drove the producers to change their mind about her fate, or if they had planned this all along is impossible to tell of course, but bringing her back is definitely a development that would make many followers of the show happy.
It’s also clear that the producers are always keeping a careful eye on the discussions of their fans and we wouldn’t be surprised if they even integrate some of the ideas they read in the show itself from time to time. This makes it a bit more realistic to expect that Grace will actually come back.
But with so much time left until the season even starts filming, the producers’ plans could easily change along the way. Until the script is finalized, we might get various conflicting rumors about the situation with Grace (and other important details of the show).
As March 2017 approaches though, we expect to start hearing some more solid information about “Peaky Blinders”. The producers will most likely not want to spoil something as major as the return of Grace if they’re really planning it, but they could definitely drop some hints about it.