iPhone 13 Pro Max battery dropped from 99% to 98% within 4 days of purchase

iPhone 13 Pro Max was released in September 2021 and discontinued in September 2022. The phone you purchased is at least 3 years old, so it would be very surprising the battery would have been at 99% on a three year old used phone in the first place. It is possible whoever sold you the phone found a way to artificially inflate the battery health on the phone. And you can expect it's health to continue to decline and possibly decline fairly quickly. When battery health dips to 80% or lower, it will then be time to pay Apple to replace the battery in the phone.

You can't compare the battery in your phone against the battery in someone else's phone. No two people use their phones exactly the same. You're going to drive yourself crazy obsessing over the chemical aging of a part, which is consumable, and will continue to degrade with every charge.

If you aren't happy with your purchase, return it to the seller. You bought a phone which was discontinued by Apple 3 years ago. It has no warranty with Apple and Apple isn't involved here.

its normal

Yes, lobsterghost1 is correct. 💯

Lithium-ion batteries will degrade over time, even when not in use. This is due to the natural chemical reactions within the battery causing self-discharge and deterioration.

All the best! 👋🏼😉