(Gnome) Keyboard shortcut to switch to workspace 3/4 - is it possible?

[Marcin Kasperski]

I am using 4 workspaces, and I got used (*) to having Ctrl-F1,
Ctrl-F2, Ctrl-F3 and Ctrl-F4 keys available to jump between them.

But ... currently, in "Keyboard Shortcuts" windows, only
"Switch to workspace 1" and "Switch to workspace 2" are available,
nothing more.

Is it possible to assign keys for "switch to workspace 3" and "switch
to workspace 4" anyhow? And, out of curiosity, where are those custom
keys saved (can't find them...).