Lancashire, who stars in both programmes, said writer Sally Wainwright is already at work penning new episodes of the popular series set in Halifax about a couple who reunite and marry after more than 50 years apart.
It had been thought there might never be a fourth series after the two-part Christmas special in 2016, but Lancashire told Graham Norton on BBC Radio 2: "I have news, we do plan to do some more this year.
She added: "It's a great show and the genius writer who is Sally Wainwright is on the case."
Lancashire also confirmed there will be new episodes of another Wainwright series, Happy Valley.
She said: "We will return with Happy Valley at some point, it will come back."
Earlier in the radio show Lancashire discussed bringing popular series back for more episodes, saying: "If you have a good piece it's great to have something that returns, as long as it doesn't return forever.
"You have to make really considered decisions about whether it should return or whether you just leave it on a high."
She added: "You make these decisions whether to stay with a drama for so many different reasons. It’s not necessary just professional, it could be personal, but I do feel alarm bells ringing when I think we have done it, we have said it all now."