How can I mount or extract TWRP Nandroid backups in Windows?
ext4.win is a posix tar archive according to the file command.

The only partitions I have in a TWRP backup that are emmc.win are boot recovery and system. Those are basically the same as boot.img, recovery.img, and system.img but renamed.

These are ext partition images and you can read the contents of them with ext2read tool.