I tried many things but to no avail (I added 'lv3:ralt_switch', 'misc:typo' to ’input-sources’ using dconf-editor). So how do I set up the 3rg and 4th level of the keyboard? I use En, Ru and Uk layouts + iBus for Sanskrit and Bengali. When I had MATE I also had troubles with setting it until I checked Miscellaneous Compatibility Options → Enable extra typographic characters. But in Xubuntu there is no such menu :(
Some keyboard layouts include 3rd and 4th level symbols, some don't. Those which do, such as the Ukrainian layout, enable the
lv3:ralt_switch option automatically, so normally that's not necessary to set manually via dconf (or gsettings).
The English (US) and the basic Russian layouts don't include 3rd and 4th level symbols.
AFAIK, the xkb-option
lv3:ralt_switch does not apply when using IBus input methods.
One way to enable extra typographic characters system wide is to open the
/etc/default/keyboard file for editing and make sure it includes the line:
Hopefully that makes a difference at next reboot.