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

sudo diskutil eraseVolume free none disk0s4
sudo diskutil eraseVolume free none disk0s3
sudo diskutil apfs resizeContainer disk0s2 0


These commands will have removed the Windows partitions and expanded the macOS partition to recover the free space. However, there will still be Windows boot files stored in the hidden EFI partition (disk0s1). These files can be removed by entering the commands given below.


  Note: If you have an legacy BIOS booting Windows, then these Windows boot files would normally not exist. In this case, the rm commands would produce an error message.


sudo diskutil mount disk0s1
cd /Volumes/EFI/EFI
rm -r Boot
rm -r Microsoft
cd ~
diskutil unmount disk0s1