Battery health drops 4% in 2 months

Over its 500 complete cycle design life span an iPhone battery can expect to lose 20% of original capacity. Depending on usage and how frequently you charge or partially charge it, it would not uncommon for people to experience anywhere from 1% to 5% over a 60 or so day period of daily use.

The battery is decaying from the moment it was assembled at the factory. You cannot stop that, and it will degrade even if the device is off and not in use. That is the nature of all chemical batteries.

Battery health is also only an approximation, and often may not be all that accurate.

See https://support.apple.com/en-us/HT208387

Yes it is normal behaviour depending on how You use the iphone.

-charge it between 20-80% of battery