UPDATE table_1 SET x1=1 WHERE table_1.x1 IN (SELECT x2 FROM table_2 UNION SELECT x3 FROM table_3 UNION SELECT x4 FROM table_4);
UPDATE table_1 SET x1=1 WHERE table_1.x1 IN (SELECT x2 FROM table_2 UNION SELECT x3 FROM table_3 UNION SELECT x4 FROM table_4);