For an n<inf>t</inf> transmit, n<inf>r</inf> receive antenna system (n<inf>t</inf>× n<inf>r</inf> system), a full-rate space time block code (STBC) transmits min(n<inf>t</inf>, n<inf>r</inf>) complex symbols per channel use. In this paper, a scheme to obtain a full-rate STBC for 4 transmit antennas and any n<inf>r</inf>, with reduced ML-decoding complexity is presented. The weight matrices of the proposed STBC are obtained from the unitary matrix representations of a Clifford Algebra. By puncturing the symbols of the STBC, full rate designs can be obtained for n<inf>r</inf> < 4. For any value of n<inf>r</inf>, the proposed design offers the least ML-decoding complexity among known codes. The proposed design is comparable in error performance to the well known Perfect code for 4 transmit antennas while offering lower ML-decoding complexity. Further, when n<inf>r</inf> < 4, the proposed design has higher ergodic capacity than the punctured Perfect code. Simulation results which corroborate these claims are presented.