I just added a new device type called "New Type", selected an icon for it, clicked OK. Closed Dude, opened Dude and it is still there. Did you do something differently? Can you post screenshots of your new device type settings?
No, I do nothing unusual - just new, name it and save in minimalistic testing.
But ... I tried to add 5 more items to device types in clean installation of Dude and it works too. Unfortunately it doesn't work in my current installation, where I have more then 300 monitored nodes. I think, it's something inherited from past settings. The same behavior I had in version 3(in my installation). I do not want to lost all my settings and history.
When I have read, that you are moving data to sqlite with Dude v4, I have been thinking, that it will be possible to add records not only through Dude GUI.