Recently we have been upgrading and migrating lots of Joomla 3 websites over to Joomla 4 and from time to time, the Joomla Update screen does not show the Joomla 4 upgrade information. In most cases, this is because the Joomla 3 Update Channel is set to the Default Channel. To check and change this, while you are on the Joomla Update screen, press the Options button in the top right hand corner and see what Update Channel you are on. To see Joomla 4, this needs to be set to the Joomla Next Update Channel.
However, in some instances we have found that even with that setting correct - the Joomla 4 update does now show.