  For an $n_t$ transmit, $n_r$ receive antenna system ($n_t \times n_r$
system), a {\it{full-rate}} space time block code (STBC) transmits
$min(n_t,n_r)$ complex symbols per channel use. In this paper, a scheme to
obtain a full-rate STBC for 4 transmit antennas and any $n_r$, with reduced
ML-decoding complexity is presented. The weight matrices of the proposed STBC
are obtained from the unitary matrix representations of Clifford Algebra. By
puncturing the symbols of the STBC, full rate designs can be obtained for $n_r
< 4$. For any value of $n_r$, 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 $n_r < 4$, the proposed
design has higher ergodic capacity than the punctured Perfect code. Simulation
results which corroborate these claims are presented.
