MIUI 9 from day by day is slowly released for new devices - in the nearest future owners of Redmi 4A will receive the newest version of this user interface. Then MIUI 9 is planned to be published for six phones: Mi2/2S, Mi 4i, Redmi Note 4G, Redmi 2, Redmi 2 Prime and Mi Note, and ROMs for these devices will be published gradually from December 2017 to January 2018.
Apart from this information we can also read on official Xiaomi's site that MIUI 9 in 7.11.16 version will be the last official version of MIUI released for these devices. In Xiaomi's opinion reduction of devices which will receive MIUI 10 will allow them to improve significantly their user interface in future.