It didn’t work, at first.
Then, I changed the $meta_query to $args (at the top). And it works! Thank you.
$args = array(
'relation' => 'OR',
array(
'key' => 'service1',
'value' => $_GET['service']
),
array(
'key' => 'service2',
'value' => $_GET['service']
),
array(
'key' => 'service3',
'value' => $_GET['service']
),
array(
'key' => 'service4',
'value' => $_GET['service']
),
)
$query->set('meta_query', $args);