You might not believe me since I don't have many posts but this jailbreak does work for 4.20. It doesn't let you sign packages yet since it's in beta stage and no packages are signed with 3.56+ keys but this jailbreak does work. I didn't believe it at first either but then I tried it and it did give me the Install Packages option after installing this .PUP.
http: // www . youtube . com/watch?v=fufVFIUMAQI&feature=plcp. See for yourself.