According to this:
http://www.sram.com/en/srammountain/...istshifter.php
the MRX is a 2:1 ratio, which means it's not gonna work with the SX4. Any index-able shimano derailer should work.
This:
http://www.sram.com/en/srammountain/...derailleur.php
lists the SX4 as 1:1
It's crazy how the manufacturers put specs on their site!