The only way mSecure 6 can know there is mSecure 4 data to migrate is if mSecure 4 is installed on the device as well, and the version of mSecure 4 is the latest release of that app which is 4.5.4. First, are you certain mSecure 4 is installed on your wife's iPhone? If you are, please open mSecure 4, tap the gear icon in the bottom right corner, then tap "About mSecure." What version does it say is installed?
Worst case is that you'll have to create an email backup in mSecure 4 on your wife's iPhone, and then you'll be able to restore from that bakcup in mSecure 6. After I hear back from you, I'll see if there's a way we can make it work through the Settings of mSecure 6 before going the route of making an email backup.
You may be able to update to the latest 4.5.4 version in the App Store app, but I'm not sure. Go ahead and open the App Store app on her phone, then tap the portrait in the top right corner. In the menu that appears, you should see a "Purchased" option. Tap that, then go to your purchases and look for the old mSecure app. If you see an upgrade, go ahead and tap the options to update. If you don't see that option, then you'll need to make an email backup, which isn't difficult.
You can use the instructions in this help article to migrate via email backup: Migrate to V6 via Email Backup