You can charge it.
Because the Mi wireless charging Bank 30W supports Qi wireless charging standard, and the Mi Mix 4 also supports Qi wireless charging standard, the Mi wireless charging Bank 30W can be used for wireless charging for the Mi Mix 4.
In addition, the Xiaomi wireless charging Bank 30W also supports 18W wired charging, and you can choose to use wired charging when you need to charge faster.
Yes, this power will be a little slower, but it is compatible.