In /my/ (for the user /Home/Dashboard) pf a git marched moodle from 3.8 to 4.0, there is a block entitled 'Recently accessed items' that has a 'Show more items' button.
Do you have any addon that modifies such things?
If you turn on debugging (all the way to developer) you might see some more info about the issue. Since 4.0 is brand spanking new .... well, what can I say ... 'omicron testing'? :|
'SoS', Ken