As far as them being a 4 piece, this is what I read from Pitchfork:
As previously reported, Sonic Youth kick off a summer tour on June 13, the same day
Rather Ripped is born. Mark Ibold, formerly of Pavement and Kim Gordon's band Free Kitten, is joining the band on bass.
Here's the Link:
http://pitchforkmedia.com/news/06-05/09.shtml
So who knows what to expect. Totally different dynamic then anything they've done before.