Yes you can use a 4TB external USB drive. See the following thread where a similar question was asked recently and where one person indicated they’re using a 4TB drive with their My Cloud.
https://community.wd.com/t/my-cloud-usb-hdd-capacity/162180
Don’t think there is a maximum capacity officially reported yet. There are issues with using certain USB hard drives but they are not related to the capacity rather how the manufacturer initially created the partition and then formatted it.
