If you open Studio One and go to Studio One>Options>General, there is a checkbox for high-dpi mode. The percentage that is reflected in the high-dpi mode is set in your operating system. If you are running Windows 10, simply right-click your desktop and go to "Display settings." You will want to drag the slider bar under the option for "Change size of text, apps, and other items. You can see the percentage in this window. Mine will only go to 175 percent and when I change this setting in Windows, it is also changed in Studio One. You may want to contact the manufacturer of your monitor and see if there is an updated driver. You will also want to ask them what is supported for that monitor. For general troubleshooting in Studio One, you can try removing your settings files and re-configuring.
1. Go to Help>Open Settings folder.
2. Copy the contents of the folder over to a folder on your desktop.
3. Close S1 and delete the contents of the settings folder with the exception of the user.license file as well as the "Extensions" folder.
4. Open S1 and go to Studio One>Options (Preferences on a Mac) to reconfigure your settings.
5. Test to see if the problem is resolved. If it is not, please feel free to copy your settings files back over from the backup that you created in step 2.
*If you are missing presets or Sound Sets after doing this, simply double-click one of your Sound Sets under Documents>Studio One>Sound Sets.
Beyond this, you may want to submit a feature request under the "Studio One 3 Feature Requests" section of answers.presonus.com. Just submit a request asking for better 4k monitor support in Studio One. You will want to check and see if the request has already been submitted.