My first idea would be to reinstall dxtory. I guess it will re-register the 4 cameras then and think that OBS should see them. I quickly checked and I could see all 4 Dxtory devices in OBS.
A second idea would be to run both OBS and Dxtory as admin to make sure they are both on the same access level. Last, but not least, you could check if you can see the Dxtory cameras in another software, Skype for example.