That's right, as OBS doesn't currently support multi-channel ASIO driver types, it will only be able to receive audio from the first two inputs of an audio device like a Clarett+ 4Pre. You will need to ensure that you connect your microphone/instrument to Input 1 or 2.
Alternatively, you could create a physical loopback to add audio from additional sources. You can use a ¼-inch jack-to-jack cable to connect a line output on your audio interface directly to a line input on the interface to create a loopback mix.
John // Focusrite UK