You have 3 ( at least ) options:
**1** Leave it as NTFS since it does exactly what you want. You will get teased by the penguinistas in the playground but they will end up working for you when they grow up so ...
**2** You can use the classic method around this.
 Change the group of the ext4 partition to plugdev ( already exists on your system ):
 Set the setgid bit of that mountpoint so that any new file inherits that group:
sudo chown :plugdev /mnt/Archive
You didn't tell us what version of Ubuntu you are using so you may have to add your wife to the plugdev group:
sudo chmod 2775 /mnt/Archive
**3** You can use use bindfs that will create a "view" of that partition that will make it look a lot like an ntfs partition.
sudo gpasswd -a wife plugdev
 Install bindfs:
 To see how this will work you can temporarily create this view:
sudo apt-get install bindfs
That looks a little funny but what you are doing is mounting /mnt/Achive to itself with a view determined by the bindfs command. All old and any new files added while this view is in effect will have read / write permissions to everyone.
sudo bindfs -o perms=0666:+X /mnt/Archive /mnt/Archive
Note: to undo this view unmount it:
If that does what you want it to do then add the line without sudo to /etc/rc.local before the "exit 0" line:
sudo umount /mnt/Archive
Note: There may be a timing issue with using rc.local where the bindfs command is executed before the partition itself is mounted. If that happens then the best solution is to create an upstart job which I can help you with if there is an issue with using rc.local.
bindfs -o perms=0666:+X /mnt/Archive /mnt/Archive
EDIT: Don't know why but I should have added that /mnt/Archive has to be auto-mounted first by adding the appropriate line in /etc/fstab in order for bindfs to do it's thing. You didn't mention if you are or are planning to have this partition mounted at boot but it is a requirement.