Her latest investigations take the form of a collection of four one-act plays at the Public Theater, under the portmanteau title “Glass. Kill. What If If Only. Imp.” Written separately over the last few years, each is pointed enough on its own: short and edgy. But together, in a splendid and surprisingly emotional production directed by James Macdonald, a frequent Churchill collaborator, they are so sharp you hardly feel them slicing your skin.