It'll be May, Metcalf, Jackson, Donnelly (in no order), and then Heidi Schreck, Bening, Nielsen, or Washington for the 5th slot. I think Schreck will get the nom (for Actress and Play).
A win could come down to campaigning. In 2017, Metcalf rode the wave of love for Doll's House to a Best Actress win. Jackson, Metcalf, and Schreck will be freshest in voters' minds because their shows are still running; they can also be at every Tony luncheon and event.
May's tricky. Her performance was astonishing, but she probably couldn't care less about winning an award (and didn't do any press for Waverly). It would also be a historic win: in the last 30 years the only person to win Best Actress for a show that closed before nominations was Cynthia Nixon for Rabbit Hole (and even that closed in early April of '06).