NEARLY FORTY YEARS after Alex Katz’s final New York retrospective, the Solomon R. Guggenheim Museum has mounted “Alex Katz: Gathering,” a profession survey encompassing eight many years of the nonagenarian artist’s masterful manufacturing. To mark the event, Artforum invited 4 painters—Sam McKinniss, Amy Sillman, Jamian Juliano-Villani, and David Salle—to mirror on the legacy of this unflagging paragon of approach and elegance.