While fans have been hoping for a new season since the show ended in 2015, Miranda Hart, the creator and star, has stated that she believes the story concluded perfectly with the ending of series 3.
However, she has also hinted that she might revisit the character in the future, potentially in a different format. So, a full series 4 is unlikely, but a special or a different kind of project featuring Miranda is not completely out of the question.
For the most up-to-date information, it's best to follow Miranda Hart's official social media accounts or check for any news from her production company.