Security Question

My bank, on their phone app, has implemented a 4 digit login without a password. It is very simple and easy to use, but I was wondering just how secure it is?
I know this is not a usual question for AI2 help, but I ask for a bit of latitude considering I may use something similar in an App I may write usind AI2.

There are likely other things going on in the background, such as your device, your phone number, your google account (?), that are then combined with this 4 digit pin. If all the stars align, then your banking app will proceed.

This must be the case, because it is obvious that other bank customers may/will choose the same pin as you, similar to a bank/credit card.