HAVANA — New pictures of Fidel Castro show the stooped but smiling former leader venturing out to meet with workers at a Cuban scientific think tank — the first to show him in public since he got sick four years ago.

Previously, Cuba had occasionally released pictures showing him in private meetings with dignitaries, most recently during a visit in February by Brazilian President Luiz Inacio Lula da Silva. In those pictures, Castro looked cheerful and robust.

The latest batch of pictures of...