Hi and welcome to Arturia forums.
Does your VST3 plug-ins show up?
If so, then keep in mind, that Arturias VST2 and VST3 plug-ins share the same ID, and in such cases Studio One give VST3 priority.
That mean you can't have both the VST2 and VST3 versions availble in Studio One at the same time. You have to either cut the path to VST3s in Studio One, to remove the VST3 versions from the VST3 folder or not install the VST3 versions, if you wan't the VST2 versions to show up.
I use Studio One 3. I guess it's the same in Studio One 4, but i don't know if Presonus have made it possible to see both versions, and if so there should be an option in Studio One 4s plug-in browser to select an option to do this. I doubt they have added this option, but please report back if they have.