Reactions of Grignard Reagent: 1,2 vs 1,4 addition
This is not a 1,4 addition.  It is a 1,2 addition across a carbon-carbon double bond.  This mode of reaction is unusual for Grignard reagents, but here a highly stabilized "double benzylic" anion is formed and the competitive reaction of adding to the carbonyl group is sterically hindered.