Re: Need help with iPod Touch 4.2.1 Jailbreak
What are you hoping to gain with the jailbreak? I might still have the old app around for the mac, if that helps.
But, it sounds like you're looking for something you won't get. You can jailbreak the device, but that won't allow your apps to run on the device. The reasons the apps aren't compatible relates to API used in their development. That basically means Apple adds tools for developers to use when they release new iOS versions. If you use one of these tools that wasn't available in earlier iOS, the older versions won't be able to understand the program.
Jailbreaking will open the device up to customization and tweaks that support older iOS. If that's what you want, then you'll see benefit. If you're solely looking to use the apps, you won't be able to perform that task.