Hello,
3-4 hours is "normal", depending on the conditions under which you use your phone. You have to keep in mind that the battery is not very big, that is the biggest weakness of the MI9SE.
Do you use Dark Mode? Is the brightness set higher than normal on your phone? How is the WLAN connection or do you use more the mobile connection?
How are the standby times at your place? Is AOD and FOD on? Wlan and BT search active in the background?
5h active screen time on average is normal for me. If I surf more and read a lot, there are more hours on the screen.
In some other threads I have already written what I have disabled to achieve higher times.
2-3 days without charging in between are possible for me.
One way to achieve the best battery life is to use the Global or EEA Stock ROM, root out all Google Apps and then use only the pure Xiaomi services.
The advantage of the Stock ROM is that the native apps such as the browser, gallery and music app are updated independently of the Playstore.