iphone 13 battery decreased by 4% in a month

If you buy a used phone, you take your chances on the battery. 4% is a really significant loss for a month's worth of operation, so this tells me that the battery was already declining quickly as a result of hard use by the previous owner....before....you purchased the phone.

Plan to replace the battery in 2-3 months when Battery Health is down to 80%

$89 (U.S.) at the Apple Store or an authorized Apple service shop.

The iPhone 13 is over 2 years old. The typical Apple Battery lasts about 2 years, so yours is due for replacement soon. Apple recommends that the battery should be replaced when Battery Health reaches 80%. You will soon need to make an appointment at the nearest apple Store for Battery Replacement. In the meantime, check this link...

Batteries - Maximizing Performance - Apple

Also this link...

Apple Store - Find a Store - Apple