Twitter publishes mock-ups of longer tweets and how they might appear in-stream.

Musk has consistently stated his intention to incorporate YouTube-like capabilities into the app, allowing users to publish lengthier movies and preventing viewers from switching to another app to consume video content. Lengthier text is another priority, with Musk informing followers that longer tweets – perhaps up to 4,000 characters – will be available shortly.
Short-form language has been a core aspect of Twitter, therefore switching to walls of text dominating the tweet feed is not an option.
Twitter designer Andrea Conway recently revealed some mock-ups of how lengthier tweets would appear in-stream, as well as how the process might operate if/when deployed.

Longer tweets would most likely be displayed in-stream at the standard tweet length, with a ‘Show more’ signal at the end of the first 280 characters. Users may then expand the tweet to 4,000 characters, which would be presented in a lengthier tweet window.
This would allow Twitter to give greater space for people to communicate lengthier thoughts. It also appears like the only way to lower the enormous tweet panel again is to scroll down to the bottom of the extended tweet – and if the character limit is truly upped to 4k, scrolling only to find out what’s buried behind that ‘Show more’ marker might be frustrating.
Twitter may address this by providing additional information about how long the complete tweet lasts.