Not only is that some spectacular mental shortcut to assume the iPhone Air will carry a M4 because it will be as thin as an iPad Pro, but what good would a M4 be in an phone?
It shortens the autonomy of the battery (which is already lightweight given the available room), and it's utterly pointless in a mobile device.
Apple is thought to launch an ultra-slim version of the latest iPhone in 2025. Its edge in terms of design might come with a major trade-off, however, according to a typically accurate leaker. The same might apply to the 17 Air's Samsung counterpart, the Galaxy S25 Slim.