Is 99% battery health normal for four a months old iPhone?

Your battery should be much closer to 97% at this time. Lucky you.

In general....(every battery is different, we are talking about averages over time).......you can expect Battery Health to decline about 1% for every 50 charge cycles.

But....averages tend to average out over time. My battery once went 3 months with no decline, then dropped 4% the next month. Battery decline will not be linear, so 2 different phones will rarely look the same on a battery performance chart.

Batteries, like people are individuals. No two people age exactly the same and batteries don’t age the same either. The exact usage, chemical makeup, storage conditions, exposure to heat and cold and about a dozen other factors mean that every battery ages differently. Battery aging is not a linear (straight line) and degrades at different rates as the chemicals in the battery age and other factors mentioned above. The graph below shows the 4 batteries that were recharged differently and look how differently the batteries capacity changed with age. There is nothing linear in the aging process.

Apple expects your battery health to be at 80% after 24 months. If your battery health misses that expectation, contact Apple Support.

Please review the Apple Support articles on battery optimization. It contains the best  current practices for battery charging.

iPhone Battery and Performance - Apple Support

Batteries - Maximizing Performance - Apple

About the battery and performance of iPhone 11 and later - Apple Support

It's normal for the battery to lose 1% of its health every two months or 50 charge cycles, or double that rate for units earlier than the iPhone 15.

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Your iPhone expected to drop about 1% battery health for every 50 charge cycles, and the decline isn't always linear. Meaning some months you will lose more than expected, and other months none at all.

After roughly 150 charge cycles, you should be at 97% battery health. But instead you are at 99%. So your iPhone battery is ahead!

Keep in mind that all rechargeable lithium batteries chemically degrade, and that when it reaches 80%, you should have the battery replaced.