NEARLY FORTY YEARS after Alex Katz’s last New York retrospective, the Solomon R. Guggenheim Museum has mounted “Alex Katz: Gathering,” a career survey encompassing eight decades of the nonagenarian artist’s masterful production. To mark the occasion, Artforum invited four painters—Sam McKinniss, Amy Sillman, Jamian Juliano-Villani, and David Salle—to reflect on the legacy of this unflagging paragon of technique and style.