Android 4.4 KitKat should be ready for release at the end of this month and until then, the folks at Android Police managed to grab this Google Play Store 4.4.21 before it has began rolling out to devices. This is not an official release from Google and may still be in a test phase, but it definitely be what we should get in Android 4.4 or maybe a bit modified.
You can download the Google Play Store 4.4.21 APK from here.
The most noticable change in this version appears to be the slide-out navigation from the left which navigates you to the home screen of the Play Store, your installed apps, shopping for apps, a quick access to your wishlist and for redeeming codes.
To install this new Google Play Store, simply save the APK file to your phone and open it. You may get a warning about installing apps from outside sources and you will be required to enable the option "Unknown sources", so do what it tells you and you will install the app. It should work flawlessly after installing the apk and no reboot is needed.