PIN Issue

My eldest daughter has just upgraded her iphone and is donating her old iPhone 7 to my youngest daughter (no SIM) . My youngest daughter had an iPod touch. My youngest daughter is attempting to sign in on the iPhone 7 with her Apple ID. The final security process is requiring her 6 digit PIN. The issue is she never had a 6 digit PIN only a 4 digit PIN so she is therefore being locked out her new phone after too many failed attempts . Can someone assist with this problem and point me in the right direction?