XIM4 is the much older model. XIM Apex is cheaper, has better device support, more memory slots, more advanced features, audio passthrough for controller chat and runs faster.
XIM4 is 125Hz only, XIM Apex is 125Hz, 250Hz, 500Hz or 1000Hz, depending on your preference. Apex also features a power port on the USB hub, in case you need to add more power to the setup.
Long story short, Apex is the newer improved version. I'm the Australian distributor for XIM Apex at
https://modsquad.com.au. Let me know if you have any more questions.
