This is not a tutorial! I was very lucky to find that custom iOS 4 ipsw for my ipod touch 4th Gen iPod Touch I created years ago, so unfortunately I can't help you with your downgrade from iOS 6 to a cooler version :/ - how ever, I hope I triggered some nostalgia :)
I found an old signed ipsw file I created back in 2013. With some tricks I managed to install my old Apps, if you want a longer video or a review to more old Apps, let me know in the comments!