How do I prevent incoming call notifications on a non-primary iPhone?
I have had this same problem, and fixed it by first turning off iCloud Drive. This fixes the shared call history between devices. Second, go to Settings -> Phone -> Calls on Other Devices -> Off This fixes calls ringing up between devices

In your case, you could also avoid the problem by turning on Do Not Disturb on the iPhone 4 — that will stop all notifications. It is found in Settings -> Do Not Disturb