Back in the days when packing them in was permissible, Brad Mehldau packed them in to concert halls worldwide. To savor, in this cheek-by-jowl piano bar, his Brahmsian impromptus and Beethovenian scowl is thus a rare privilege indeed. If you’re not brisk enough (or Delta-defiant enough) to grab a seat for one of the four shows—Monday’s brace solo, Tuesday’s with “special guests”—you can catch any and all online. —E.E.