The first MIUI 13 build is expected to be rolled out in a beta form very soon for users of some Xiaomi smartphones with the China ROM. While it was widely expected that the upcoming Xiaomi Mi Mix 4 would be one of the first devices to operate MIUI 13 out of the box, it now seems the Mi Mix 4 will come with MIUI 12.5.
https://www.notebookcheck.net/First-MIUI-13-build-incoming-but-looks-like-the-Xiaomi-Mi-Mix-4-will-be-running-on-MIUI-12-5-out-of-the-box.549243.0.html