Hi,
It depends whether you have the global version or the china version.
I would bet given the experiences of many recent posters who have the R4AC (100Mbit) that you have a recent international version. In that case, this image is what you need
Xiaomi R4AC - Problems installing OpenWrt - #3 by TheCheshireMan
Check out this thread for further information on determining the variant of R4AC that you have:
Xiaomi Mi Router 4A - R4AC New revision
Cheers,
- Mark