The OpenELEC project just announced that OpenELEC 4.0 Beta 2 has been released.
If you are currently running the 3.2.4 version or earlier it is recommended that you perform a clean installation. This is because the XBMC 12 (Frodo) and XBMC 13 (Gotham) configurations are not totally compatible with each other.
If you are already running OpenELEC 4.0 Beta 1 the most simple way to upgrade is to go to System - OpenELEC - System - Check for updates now and let OE update itself. Super-easy and quick.
This version runs very well on Haswell NUCs. My only issues are with live TV when watching HD channels, but this might be due to the tuner I'm using (PCTV 290e). I've tried different backends, different frontends, OE and Ubuntu Linux and no matter what, it gets pixelated every now and then. Especially so, if I open the menus on top of the live TV screen.