In this paper we study the following coupled Schrödinger system, which can be seen as a critically coupled perturbed Brezis-Nirenberg problem:Here, ⊂ R 4 is a smooth bounded domain, −λ 1 ( ) < λ 1 , λ 2 < 0, μ 1 , μ 2 > 0 and β = 0, where λ 1 ( ) is the first eigenvalue of − with the Dirichlet boundary condition. Note that the nonlinearity and the coupling terms are both critical in dimension 4 (that is, 2N N −2 = 4 when N = 4). We show that this critical system has a positive least energy solution for negative β, positive small β and positive large β. For the case in which λ 1 = λ 2 , we obtain the uniqueness of positive least energy solutions. We also study the limit behavior of the least energy solutions in the repulsive case β → −∞, and phase separation is expected. These seem to be the first results for this Schrödinger system in the critical case.