The bit error rate performance of bit-interleaved turbo-coded modulation (BITCM) strongly depends on the M-ary modulation which is employed in association with the binary turbo code. When the parameter M is a power of 4, we show that classical rectangular QAM modulations are particularly attractive for designing power-efficient BITCM's. We also investigate the influence of the interleaving and the mapping on the performance of BITCM's based on such modulation schemes.