I want to updated my iOS to 15.4

Go to Settings > General > About

What does it say for iPhone Model and Software Version?

If your iPhone is an iPhone 6/6 Plus or older, then you cannot upgrade beyond iOS 12

You need an iPhone 6s/6s Plus or newer in order to run iOS 15..

You need an iPhone 8/8 Plus or newer in order to run iOS 16..

Supported iPhone models - Apple Support

have you read thru this support article for updating?

Update your iPhone, iPad, or iPod touch - Apple Support

need more storage for update?

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

issues?

Updating iOS: common issues and solutions - Apple Community

video..

flores260 Said:

"I want to update to iOS 15.4"

-------

You Can't. Only the latest iOS version is installable. iOS 15.4 is no longer an option.