My iPhone battery dropped from 95% to 91%

You are NOT writing Apple. Tim isn't here, nor will he see your post. This is a user to user only forum, which no one from Apple participates. Giving other users your Apple case number is meaningless and won't help you at all.

Answer the following:

1. How many months have you owned your iPhone
2. Open Settings > Battery > Battery Health and tell us how many cycles are shown

On average, you can expect to see about a 1% drop in battery health per month of ownership. Or, for every 25 cycles, a 1% drop. Battery Health decline is not linear. It can remain steady for a while, then drop a few points. This is normal and expected and battery Health goes ONLY one direction and that is DOWN. All batteries lose a little health with every full cycle charge.

There is likely nothing wrong with your battery and Apple will NOT replace a battery in an iPhone until it's health dips to 80%, which it absolutely will do at some point in the future.

I was told to downgrade the ios but would that get my 4% back that I lost in a day?

No one at Apple Support would have told you that, since this is not supported.