load commerce coupon
Sounds like the perfect job for an EntityFieldQuery:

$query = new EntityFieldQuery();
$query-&gt;entityCondition('entity_type', 'commerce_coupon')
  -&gt;fieldCondition('commerce_discount_reference', 'target_id', 4);

$results = $query-&gt;execute();
if (!empty($results['commerce_coupon'])) {
  $coupons = commerce_coupon_load_multiple(array_keys($results['commerce_coupon']));
}