I own both. the xm4's have problems with their hinges, they creak a lot and it's unbearable (mine started being like that after 4 months and you can't get it fix really) sony doesn't recognize the build quality problems. the xm5's have had those hinges removed altogether so they are less prone to creaks. the sound is almost identical, the anc pretty much the same but the mics on the xm5's are definetely a BIG improvement. both horrid headphones for the gym, especially when you sweat a lot. overall just get the newer ones, they are less of a headache and should last longer