Hi, here is our standard reply on this question:
The LCD-MX4s have the easy-to-drive diaphragm and voice coil of the LCD-X combined with the 1.5 Tesla magnetic force of the LCD-4.
The MX4 is the evolution of the X, and was developed specifically for the pro market based on feedback from many engineers we've partnered with. Externally it features the lighter magnesium housing and carbon fiber headband for greater comfort over long sessions.
Internally, the MX4 also uses our
Double Fluxor magnet array, which more than doubles the magnetic flux focused on the diaphragm, and our
Uniforce voice coil, which distributes the flux more evenly for the ultimate in "pistonic" motion of the diaphragm. This gives greater control over diaphragm movement, resulting in even faster transients, lower distortion, and better imaging, and makes it our most efficient LCD headphone.
The two carry a similar overall sound signature which is very neutral and detailed, but the MX4 is definitely more refined, and most astute listeners prefer it over the X. Which is better for you is a personal choice and we recommend trying them for yourself, but hopefully this gives you enough to go on to make an educated choice.