Bootcamp to generate Windows 10 USB key - Needs NTFS partition
To change the drive partitioning to a pure GPT format, enter the following commands. The commands may not make any sense to you, but the volume format can change the type of partitioning. 

sudo  diskutil  erasevolume  jhfs+  BootCamp  disk0s4
sudo  diskutil  erasevolume  FAT32  BOOTCAMP  disk0s4


From the description of your problem, below is what you need to do when you are asked about partitioning while installing Windows.


Select the partition labeled BOOTCAMP.
Click on the button labeled "Format". The BOOTCAMP label should disappear.
Proceed to install Windows.


You have assumed the Boot Camp Assistant application can NTFS format a partition. If fact, it can not. Instead, the partition is FAT32 formatted with the label BOOTCAMP. You must click on the Format button during the Windows installation to NTFS format the partition. This removes the BOOTCAMP label. Later, when the Boot Camp Support Software is installed, the NTFS partition will be labeled BOOTCAMP.



To be clear, Windows installations on this model Mac must use the EFI boot method. This requires the installation drive to contain partition entries in only the GUID Partition Table (GPT). Both Windows and macOS (OS X) will only see the the GPT entries. Therefore, both Windows and macOS (OS X) with think the drive is GPT partitioned.