As of October 2018, SmartThings is in the process of transitioning between the SmartThings Classic app and the newer SmartThings app. You'll need to install the custom device handler for the sensor regardless which app you're using, but just know that you'll only be able to customize the sensor using the Classic app with the handler. The new SmartThings app currently doesn't give access to any device's advanced settings. Basic functionality will be there, however.
If you haven't added a custom handler before, we got you. The handler is a piece of code that enables the sensor's advanced settings and makes sure the device is properly configured. The process takes less than 5 minutes and you can use the steps found here.
Once the handler is installed and shows up in your IDE portal, you can include the 4-in-1 sensor to your network. Here's how to do it with the Classic app:
Once the sensor is set up in its intended location, it's not a bad idea to perform network repair to optimize connectivity. This will help SmartThings find the best communication route to the sensor. Here's how to repair the network.
Here's how to include the sensor using the new app (once the handler is installed):
If you're having issues including your sensor to SmartThings, we'd recommend excluding the device using the instructions here.
If you have any questions about the process or you're experiencing issues, let us know!