Just noticed that QTS 5.2.10.3568 Build 20260722 is available on the support site. It's not showing up as available from inside QTS firmware update menu just yet. Will be testing this shortly on a TS-664 and TS-464. Release notes are here

Does anyone have a Retroarch installation that works on HD Station 4? I have looked everywhere for the compatible download and can only find the one that needs HD Station 5.