I bought 4 week ago iphone 11 and the battery health is going down 100 to 97 whats the problem

There is no problem. Normal battery function. When it gets to 80% or less, get it replaced.

Is your iOS up to date? (currently 14.5.1)

About recalibration of battery health reporting in iOS 14.5 - Apple Support

Hi,

This should help you to understand how is battery health working, how to maximize battery life and detect if there’s any problem with your battery.