How to resolve Bootcamp failed installation error
The Boot Camp Assistant attempts to create a temporary internal FAT32 formatted volume containing the files from the Windows 10 ISO and the Windows Support Software downloaded from the internet. However, since the ISO contains a file named install.wim in the sources directory which is over the 4 GB size limit for FAT32 volumes, the Boot Camp Assistant fail to complete this task.

You reference a solution where this file would be broken down into smaller files. However, there is a possibility that this solution alone would not be sufficient. If the internal FAT32 formatted value was to small in size to contain all the files in the latest Windows 10 ISO, then the Boot Camp Assistant would again fail.

The obvious solution would be to upgrade to a newer version of macOS, where the volume would be exFAT formatted and sufficient in size. Beyond the obvious, here are the steps I would suggest following.


Use the Boot Camp Assistant to download the Windows Support Software.
Shrink the macOS partition and create a new ExFAT formatted partition labeled BOOTCAMP.
Erase a 16 GB or larger flash drive. Use ExFAT format and a Master Boot Record (MBR) partitioning scheme.
Mount the ISO. Copy all the files to the 16 GB flash drive.
Copy all the Window Support Software files to the 16 GB flash drive.
Boot the mac from the flash drive. Look for EFI Boot in the Mac Startup Manager. My suspicion is that this will fail because your Mac has not had the firmware upgrade provided by installing High Sierra. If this happens, proceed as follows.
Erase a 8 GB or larger flash drive. Use MS-DOS (FAT) format and a Master Boot Record (MBR).
Copy all the files, except for the sources\install.wim file, from the 16 GB flash drive to the 8 GB flash drive.
Boot from the 8 GB flash drive and install Windows 10 to the BOOTCAMP volume. (You will have to format this volume before proceeding)
A some point during installation, the Mac may boot back to macOS. If this occurs, select Windows as the startup disk in System Preferences, then restart the Mac. Installation of Windows will continue.


The method relies on booting from the FAT32 formatted flash drive and having the Windows installer automatically find the install.wim file on the ExFAT formatted flash drive. It seems improbably that this would work, but it did when testing in VirtualBox.