The key assumption in all space-time block coded orthogonal frequency division multiplexing (OFDM) schemes is that the channel remains static within the whole codeword length. This will inevitably introduce an irreducible error floor, caused by the inter-transmit-antenna interference (ITAI), in the high signal-to-noise (SNR) region over the time selective fading channels. To mitigate the impact of ITAI, a novel signal detection scheme for four-transmit-antenna space-time-frequency coded OFDM system has been proposed in this paper. Through the theoretical analysis and computer simulations, it is proven that the proposed method can handle both slow and fast fading channels very effectively as well as efficiently, removing the error floor within the normal Doppler frequency range .