To everyone posting on here:
The "Joomla Next" update channel will report the availability of Joomla 4.0.x and extensions; useful for seeing which of your extensions are ready for J4 and which are not, but please be completely sure that all your extensions are compatible before hitting the "Update" button
Remember to set the channel back to "default" after you've seen what's what! Leaving it on "Joomla next" means that the checker will run each time you have something to update.
In Joomla 3.10.x, using the "Default" channel, the Update pre-checker will show when there are J3-series updates available. If it doesn't show and there are no updates available then this is expected behaviour.