i am locked out of my moms old iphone 4 and neither one of us know the passcode

my iphone 5s completely broke yesterday so today we went to the sprint store to get an iphone 6s as an early birthday present but they had to preorder it and it wont be here until next week so I am stuck using my moms old iphone 4s. the worker at the sprint store registered my number to the phone so I can see texts and calls coming in but I cant unlock the phone to see who they're from. when I try plugging the iphone into itunes on my computer to restore the phone it wont even register that the phone is plugged in until I can allow access from the phone itself which is once again impossible for me to do since I don't know the password. my birthday is tomorrow and im desperately looking for anyway to get into the phone. im beyond frustrated and just need someone that can help. please answer quickly!