What’s it like when the first four female Supreme Court Justices get together to sit for one oil portrait? “Semi-controlled chaos,” artist Nelson Shanks told us. The painting, unveiled Monday at the National Portrait Gallery, took not quite eight weeks to complete, and involved a very “upbeat” four-hour portrait session with the justices all talking and joking. “They’re tremendously good friends,” said museum director Kim Sajet. “They joke around a lot, and they respect each other a lot.” Sandra Day O’Connor, Ruth Bader Ginsburg, Sonia Sotomayor and Elena Kagan are scheduled to attend a private gathering Monday night at the gallery to celebrate the painting, which was first commissioned by art collectors Ian and Annette Cumming about two years ago. And no, it’s not a coincidence that O’Connor and Ginsburg are the ones seated on the couch, in a room based on the Supreme Court Building — Sotomayor and Kagan are the relative newbies, so they had to stand.