“Sleepy Hollow” after all is a TV show that brings characters from the dead. It could be recalled that Ichabod Crane was raised from the dead and so was the Headless Horseman during Season 1.
A lot of the characters of the show since the first season have come back from the dead time and again including Capt. Frank Irving of the Sleepy Hollow Police Department, played by Orlando Jones.
So it is possible that Season 4 would just surprise fans with the return of Abbie Mills from the dead through some supernatural means. The producers and showrunner know the value of Abbie Mills in the TV series and those fans rooting for ‘Ichabbie’ so killing her off just like that simply would not make sense for a show that has been struggling to get its fine form back after a rather mediocre Season 3.
Incidentally, Tom Mison has recently disclosed that some scenes of the Season 4 of “Sleepy Hollow” might be filmed in Washington, the American capital.
It could be recalled also during the final scene of the Season 3 finale that Ichabod Crane was surprised when men in suits approached him while he was paying his respects to the grave of Abbie in the cemetery. The men claimed that they were members of Washington’s secret society dedicated to deal with supernatural events.