The startup disk cannot be partitioned or restored to a single partition. Windows 10 and Mojave iMac Fusion Drive
This is a very common question. Again, here is the commands you need to enter in a Terminal application window.

The next two commands will remove the unlabeled "Window Recovery" disk1s4 partition and the "Microsoft Basic Data" disk1s3 partition labeled BOOTCAMP 

sudo diskutil eraseVolume free none disk1s4
sudo diskutil eraseVolume free none disk1s3


The next command return the free space back to the disk1s2 APFS container partition.

sudo diskutil apfs resizeContainer disk1s2 0