I just built mine with the YZ2 front aluminum and YZ2 rear aluminum hangars (and pills). Like was mentioned previously you will have a 2mm gap between the hangar and the stock yz4 ballstud mount. I looked at my stock plastic YZ2 ballstud mount and it is actually a little thicker horizontally. So I used the YZ2 ballstud hangar plus the YZ4 1mm plastic spacer it comes with along with the YZ2 option steel RR hangar and that combination fits perfect.
The only thing though, is that the swaybar retainers (little plastic things) on the YZ2 and YZ4 are different. I'm going. No away bars right now for a small-medium indoor clay track.