The htmx 4.0 Changelog

fetch() has replaced xhr as the core AJAX infrastructure
The default swap strategy has been changed to outerHTML. You can configure this back to innerHTML to avoid pain.
CSS Transitions have been removed in favor of View Transitions
- There is no longer a swap & settle mechanic, so options relating to those are now removed
Inheritance is now explicit, via the :inherited suffix on attributes
- Thus, inheritance-related attributes like hx-disinherit have been removed
hx-disable has been renamed to hx-ignore
hx-disabled-elt has been renamed to hx-disable :/
History no longer caches any local content, instead a full page request is made and the body tag is swapped out
The queue option was dropped from hx-trigger, should use hx-sync instead
Extended selectors with spaces in them (e.g. next .foo) must now be enclosed in quotes for the from option on hx-trigger (e.g. hx-trigger="click from:'next div'", rather than hx-trigger="click from:next div")
hx-confirm now can can return a string to confirm or, via the hx-confirm='js:confirmMe()'' form, use a Promise-based callback
hx-on:* now only follows the hx-on:<event name> standard. Because JSX now supports colons in attribute names, there is no special handling. There is also no special handling for htmx events: all events should be fully qualified
hx-sync no longer supports the abort option