How to get iOS 15.4 in my iPhone?

iOS 15.4 is not yet available to the public - 15.3 is the highest you can update to right now. If you are talking about becoming an iOS beta tester, see this link on Apple's beta program: https://beta.apple.com/sp/betaprogram/

Caution: I would not recommend using beta software on your main iPhone (use a second iPhone if you have one). Otherwise, wait until iOS 15.4 is released.

iOS 15.4 is in beta testing phase. It is not available to the general public yet.

Beta software is expected to have bugs and glitches, some device breaking or bricking and should not be installed unless you know exactly what you are doing and are prepared to handle said issues on your own. There is no support from Apple for Beta versions for regular users, only for registered Developers.

iOS 15.3 is the currently released and supported iOS version.

iOS 15.4 has not been released by Apple yet.

Wait until it is released.

some devices don't indicate face ID for mask and glasses.

dreavsandrea wrote:

some devices don't indicate face ID for mask and glasses.

That’s certainly true. After all, not all devices offer Face ID.