This is a common problem for KDE newcomers. This is because of default value in KDE Global Keyboard Shortcuts. By default "Ctrl + F4" keyboard shortcut assign to "Switch to Desktop 4" action in KDE Global Keyboard Shortcuts and then if you have 4 virtual desktop (or more) in your KDE desktop, "Ctrl + F4" bring you to 4th desktop and if not it does not do anything !
To solve this problem (if it is a real problem for you like me), you have to assign another keyboard shortcut to "Switch to Desktop 4" action:
- Open KDE Global Keyboard Shortcuts program.
- From KDE component combo box select "KWin".
- In the search text box type "switch to desktop".
- Finally, in the search result lest, reassign another keyboard shortcut to "Switch to Desktop 4" action.