Viewers would see a new leading lady to co-star with Ichabod Crane (Tom Mison) in Sleepy Hollow Season 4 as Nicole Beharie's character FBI Agent Abbie Mills died in the Season 3 finale 

Mills died on Sleepy Hollow Season 3 after sacrificing her life in the line of duty to bring down Pandora. Fox Entertainment President David Madden said that Mills was one of the foundations of the show and that it was very hard to decide on a fourth season without her.

Although the decision to renew Sleepy Hollow without Mills was hard, Madden said that the pitch for Season 4 was hard to say no to as he said, "[Sleepy Hollow creatives] pitched us an array of new characters and new situations for Crane to get into and the first seven scripts are fun. We think there's a lot of good new stories to tell, according to Cinema Blends

Producers and creators of the series, however, decided to renew the show without Mills as the Sleepy Hollow Season 4 will start anew. 
GameNGuide reports that Sleepy Hollow Season 4 added several characters including a new female lead that will be played by Janina Gavankar. She will play a special agent for Homeland Security and a single mom named Diana.

Moreover, Diana and Ichabod reportedly have clashing different approaches in missions but will find a surprising connection despite these differences.

Other characters that would join Gavankar, according to International Business Times are Jeremy Davies in a villainous role, Ona Yaffe playing Diana's daughter, Rachel Melvin as engineering prodigy Alex Norwood, and Jerry MacKinnon as Jake Wells who catalogs supernatural phenomena in Washington.Lyndie Greenwood's Jenny Mill is an exception who will remain for the season 4.

In the similar report, the setting of the supernatural series will move from New York to Washington. 

Sleepy Hollow Season 4 is set to air midseason on Fox.