Although the last season finished up in December of last year, there's still no official word on when the next season will kick off beyond a general idea of 2019.
However, in an interview with the BBC, showrunner / creator Steven Knight confirmed that he's currently working on the fifth season and that he has a sixth and seventh season planned for the future as well. Previously, Knight had said that he intended the series to end after the fifth season, but it now looks like it's going longer than expected.
On top of that, Knight confirmed that Mr. Thomas Shelby himself is on for it as well. "We've talked to Cillian Murphy and he's all for it, and the rest of the principal cast are in for it," confirmed Knight. The last episode of the fourth season saw Shelby become elected as an MP, and the fifth season will likely follow on from this.
Murphy was recently linked to rumours that he may be taking on the role of James Bond, but told us in a recent interview that it's "not on his radar."