Here are my thoughts:
3 Weeks is a long time for 4 European cities.. You also dont want to leave London back to the USA, you want to make it your first stop (due to very high taxes).. How about something like this?
You'll get to see London, Paris, Berlin, Rome, Venice, Mykonos & then fly back to SFO via ZRH
I dont know how updated the YQ thread is but being that this is in June there is no reason why this cant be done in style.