Abstract. We consider both local and global theta correspondences for GSp 4 and GSO 4,2 . Because of the accidental isomorphism PGSO 4,2 PGU 2,2 , these correspondences give rise to those between GSp 4 and GU 2,2 for representations with trivial central characters. In the global case, using this relation, we characterize representations with trivial central character, which have Shalika period on GU(2, 2) by theta correspondences. Moreover, in the local case, we consider a similar relationship for irreducible admissible representations without an assumption on the central character.