As you mentioned, an X-Brick is an HA component of the array with no single points of failure. When you say “X-Brick goes down” - you have to think about which failure scenario you are referring to – is it a power failure, storage controller failure, IB switch failure, SAS link failure, etc? The XtremIO array would be able to overcome all of those failure scenarios (and more). An X-Brick "going down” is very much like a dual controller storage array “going down”. Do think about what happens to the data when a dual controller storage array “goes down”?  To answer your question, we do spread our data evenly across all the SSDs and the X-Bricks in the array for best (and consistent) performance and high SSD endurance. We do need all X-Bricks in the array to be available for the data to be available.
We know all hard ware is reduandant in a Xbrick.  But, What happens when Xbrick goes down in a  4 X-Brick cluster in worst case scenario for any reason ?  Is that cause a whole cluster data in accessible ?
What happens when one XBrick goes offline