Hi Roman -- that 4-digit code is your pairing code. Make sure you've downloaded the Striiv app, then create an account in the app and begin the pairing process as prompted by the app (you'll start by choosing your device from a menu of all the Striiv devices).
The app will eventually ask for your pairing code, and this is when you'll enter E4F3.
Make sure your Bio is in the charging cradle and plugged in, and that your phone's Bluetooth is turned on and you're on a reliable internet connection. If you have any further issues, let us know at firstname.lastname@example.org!