Curious... will all 4 bishops be there?
I don't think so.
Firstly, it's a meeting of the superiors according to the press.
Secondly, Bishop Williamson is virtually not existing anymore for Menzingen, Stuttgart, etc.
For example, Bp Fellay's first assistant Fr Pfluger who's Krah's mentor and protector, visited the British SSPX district in March 2011. The German SSPX newsletter May 2011 published a photo with the following subtitle:
In March 2011 the SSPX first assistent Fr Niklaus Pfluger visitied the priests of Great Britain However, the photo didn't show Bishop Williamson. So he's virtually not existing for the SSPX anymore. (Fr Schmidberger is the German district superior and before him Fr Pfluger was. See
PDF May 2011, page 26, for the photo and the subtitle.)
That's different even to the freelancing priests who work for some SSPX districts whilst not being members of the fraternity. On district meeting photos they're usually shown however.
P.S.
Bishop Tissier de Mallerais is supposed to be there aswell, he is theologically comparable to Bishop Williamson.
Theoretically yes. Unfortunately however, practically Bishop Tissier doesn't dare to even uphold his script "The Faith Imperilled by Reason" about B16's heresies, because Menzingen forbid to criticise B16.
However Bishop Williamson does brings practically four (or five!) recent ECs explaining the script in details for the laity. What a brave shepherd! He knows that the souls vitally need truth and clarity, not half-truths and doubletalk, and that for a true priest or bishop to ignore that fact would be false obedience. God bless him!