Increase number of workspace with shortcuts on Ubuntu 18.04

First, you will need to increase the number of static workspaces. You can do that on the "Workspaces" tab in GNOME Tweaks (not installed by default). You can also use the terminal to change that configuration:

gsettings set org.gnome.mutter dynamic-workspaces false
gsettings set org.gnome.desktop.wm.preferences num-workspaces 6

These commands disable dynamic workspaces, and set the number of workspaces to 6.

You then can set hotkeys to switch to any workspace including workspaces higher than four, using dconf-editor, or, much more convenient in this case, using the terminal.

gsettings set org.gnome.desktop.wm.keybindings switch-to-workspace-5 "['<Super>5']"
gsettings set org.gnome.desktop.wm.keybindings move-to-workspace-5 "['<Super><Shift>5']"

These two comments will set up Super+5 to move to workspace 5, and Super+Shift+5 to move a window to workspace 5. Replace workspace-5 by workspace-6 and higher to set for higher