Asking for 6 digit passcode saying it was the one I use to access my phone, account, apple pay?

Hi, my Iphone 8 plus recently shut off and wont turn on. I have done force resetting and left it charging, haven't dropped it or anything and I was using it when it turned off. Well, I found my sister's old iphone 6 and decided to log into it so I can have a phone to communicate with in the meantime but when I logged into the Icloud, (phone was factory reset for 2 years) it asked for the 2 step verification. I had to put my sim card in her new phone for the code and it let me through. However then it asked me for a 6 digit passcode saying it was the same one I use for my phone, to access the account online, my apple pay, and so on. The only problem is that my passcode is only 4 digits long. I logged into my icloud online and then i went to the manage your apple id online. Nothing there about such thing. I ended up removing my 8 plus from my trusted devices in hopes that it would maybe help but it didn't. It still asks for a 6 digit passcode. Please help.