I finally figured it out! And corrected my embarrassing mistake in the function above…
add_action( 'gform_user_registered', 'save_additional_user_meta', 10, 4 );
function save_additional_user_meta ($user_id, $feed, $entry) {
$case_worker = rgar( $entry, '4' );
update_field('field_636957e952b06', $case_worker, 'user_'.$user_id);
}