Given how important Fury has been to the MCU (especially in Phase One when he served as one of the primary connective threads for those movies), it’d be strange for him not to appear in Avengers 4, the movie that’s concluding Phase Three and launching a new era for the franchise.
The last time we saw Nick Fury in the MCU was at the end of Avengers: Age of Ultron, where he was helping establish the New Avengers facility in New York following the evacuation of Sokovia.
Since then, his whereabouts haven’t been addressed, so he could be doing anything from continuing his spy work to relaxing in Tahiti.
And since Avengers 4’s main shoot is happening until the end of the year, there’s still plenty of time for Marvel to bring Samuel L. Jackson in to film a few scenes.