In this paper the simplest quasi-3D HSDT with four unknowns is presented. The elegancy of this theory is on its simple displacement field which is similar to the one developed by Reddy but with thickness stretching capabilities, i.e. quasi-3D nature, and just four unknowns. For validation, the principle of virtual work and Navier-type closed-form technique were utilized to formulate and solve the bending problem of functionally graded plates. The results are in concordance with the ones found in the literature. Consequently, it is recommended to be utilized in several physical and case study problems.