Joanne Chang tweets today that Flour Bakery will open its fourth location at 131 Clarendon Street in the Back Bay.     The new location of Flour Bakery is expected to open by the end of this year in the building that used to house the Hard Rock Cafe. Flour currently has locations in the South End, Fort Point, and near Central Square in Cambridge      This will be great for this area, with its numerous offices and residential units nearby.   The new Flour will also be located steps from Back Bay Station.