Do you have SHSH blobs saved?
He said iPod touch 4, which is possible to downgrade if he has the shsh blobs to the firmware he wishes to downgrade to.
If you don't know what those shsh thingies are you cannot downgrade.
Nope, I didn't. Anyways, I used iSHSH¡t and the oldest SHSH I have is 5.1.1. Saved it and sent it to my computer, so it seems to have worked well. Already downloaded Redsn0w and currently downloading 5.1.1 IPSW.Cydia is not 100% reliable. Did you back them up using TinyUmbrella?
Let us know how you make out.