Lenovo's Vibe OS is an extensive customization of Android, in this case Android 5.1 Lollipop. It's mostly familiar enough so that new users don't get lost, but it's different from what we saw on the K3 Note.
The Lockscreen is largely unchanged. You get the time and notifications in the middle, plus Call, Unlock and Camera shortcuts on the bottom. Using the Theme Center you can get a clearer screen, without all the notifications or shortcuts.
Obviously, you can use the fingerprint reader to secure the phone. In fact we'd recommend it - if not for the security, then for its speed of unlocking the phone.
The reader is always active, all you need to do is place your finger on it and the phone wakes up (bypassing the lockscreen). It's accurate and fast and your index finger naturally reaches for the reader. As an added bonus, this system is ambidextrous - the Power key may be on the right, but the fingerprint reader can be used with either hand.
There are a few more shortcuts that speed things up. When the phone is locked, you can double tap a volume button to launch the camera and take a photo. If you just want to check the notifications, you double-tap the screen. Or you can use the fingerprint reader to take a photo (taking a selfie is much more comfortable with a rear-mounted shutter key).
The fingerprint reader does other tricks too. You can use it as a Home button or as an App switcher button when the phone is unlocked. You can even use it for both, setting one as the tap action, the other as the tap and hold action.
We mentioned themes, the Vibe K4 Note supports them, though they are fairly basic - they change wallpapers (on the Home and Lock screens), app icons and notification sounds.
From here you can also change the App switcher style - from the default Android one (3D rolodex) to an Apple-style horizontal list.
The notification shade has a handy shortcut to view past notifications, in case you dismiss one but need to check it again. You can change the number of columns of quick toggles - fitting more on the screen or getting bigger tiles. Another option lets you add your agenda into the shade.
An interesting option, the Notification Assistant, keeps track of how many notifications each app has published and how many you've read. From there it allows you to quickly mute apps when they get spammy.
If the 5.5" screen is too big for you to use with one hand, you can shrink it with a gesture (draw a "C" on the screen) and use Wide Touch - a floating button, that opens a menu with several helpful shortcuts.
The Lenovo Vibe K4 Note may not have the latest Android Marshmallow, but like with many other Chinese-made phones, it doesn't stop it from offering the user fine-grain permission controls. You can view all apps that require a specific permissions or all permission for a single app. And, of course, you can individually disable and enable them.
Secure Zone allows you to create two virtual "zones," for example to keep your work and private life separate. Each zone can have its own selection of apps, accounts, and passwords. If a notification arrives in the currently inactive zone, it will show up with a red dot in the status bar. You need to switch zones to be able to view it (which means others can't do that without your permission, even if they have your phone).
Lenovo goes a step further - your data is protected even from the apps themselves. App data and documents are kept separate between the zones though there's an OpenUserData folder that can be accessed from both zones. Data in the Secure Zone can only be accessed by Secure Zone apps. Call logs and messages are shared between the zones so that normal phone operation is unaffected.