pass code problem

hello

I have an ipad 2 and I installed ios 9 public beta 1 and changed my pass code to a 6-digit one. I forgot to update it so it expired. In recovery mode I choose update button and installed ios 8.4.1. Now it requires me a four digit pass code. I entered the previous pass codes also first and last part of 6-digit pass code but it doesn't open.

I don't want to restore my ipad because i remember my back up is for ios 9 and maybe I could not recover it on 8.4.1 and also I can't update to ios9 any more from itunes.

what can I do?

thanks