We propose a new block time-reversal conjugate based quasi-orthogonal space-time block encoding and decoding (QO-STBC) scheme for a wireless system with four transmit antennas in frequency selective fading channels. The proposed scheme effectively combats channel dispersion and frequency selectivity due to multipath, yet still provides full diversity gain. We also propose a maximum likelihood (ML) solution, with a simple decoupled structure, to decode the block time-reversal conjugate QO-STBC symbols at low complexity for channels with short delay.