A lot of work has been done to make the upgrade process much easier than the 2.5 to 3.x upgrade. There is a pre-check that helps to identify issues before upgrading. This pre-check will show the extensions that need attention and even those that no information is available for.
Many of the extension builders have kept pace with the Joomla4 development making new releases that are compatible available quickly after the Joomla 4 release.
There is also Migrate Me 4 from PHP Web Design. This tool creates a clone of the site, updates it to Joomla 4 and if you're happy with the results, copies it back to the live site.