I’m pretty sure there’s no difference between internal and external ext4 (at least how gnome disks handles it), so I think it’s just trying to make sure users don’t freak out when they format it as ext4 and think their data is all gone on Windows.
Also when it’s grayed out you usually just have to install the fuse driver and file system tools, IIRC for exfat you install exfat-fuse and exfatprogs.