IIRC once you mount an ext2/3 volume as ext4, you can't go back.
ext4 has made a lot of improvements, but has also had a lot of corruption issues, even if corner cases. If you don't need extremely large files or extremely cluttered directory structures, you don't _need_ ext4, though I haven't compared directly in a few years, and I suspect a lot of these stability issues have shaken out.
The worst I experienced was kvm guests with ext4 filesystems stored in files on ext4 filesystems causing the not only the guest, but often the _host_ filesystem to go corrupt. Obviously, using LVM volumes or basically anything but filesystem images for production VMs would have avoided this, but the infrastructure I supported at the time already had a lot of this in place and too many other problems to solve to focus on it.
If you actually mount it as ext4 and enable the new features, yes. However, if you mount ext2 or ext3 using the ext2/3 implementation in the ext4 driver, that doesn't break backward compatibility with ext2/3.
ext4 has made a lot of improvements, but has also had a lot of corruption issues, even if corner cases. If you don't need extremely large files or extremely cluttered directory structures, you don't _need_ ext4, though I haven't compared directly in a few years, and I suspect a lot of these stability issues have shaken out.
The worst I experienced was kvm guests with ext4 filesystems stored in files on ext4 filesystems causing the not only the guest, but often the _host_ filesystem to go corrupt. Obviously, using LVM volumes or basically anything but filesystem images for production VMs would have avoided this, but the infrastructure I supported at the time already had a lot of this in place and too many other problems to solve to focus on it.