Problem with nested UPDATE queries
Try 

UPDATE Tab1 JOIN Tab2 ON Tab1.b = Tab2.b
SET Tab1.a = '3'
WHERE Tab1.c = '4'
;


Check the documentation for further information (look for 'multiple-table syntax').  An important point is:


  If you use a multiple-table UPDATE statement involving InnoDB tables
  for which there are foreign key constraints, the MySQL optimizer might
  process tables in an order that differs from that of their
  parent/child relationship. In this case, the statement fails and rolls
  back. Instead, update a single table and rely on the ON UPDATE
  capabilities that InnoDB provides to cause the other tables to be
  modified accordingly. See Section 14.3.5.4, “FOREIGN KEY Constraints”.


You may check a similar example on SQLFiddle.