my ipod touch 4g (4.2.1) buttons (sleep and volumes) stops working and i tried to use Erase All Content and Settings option! but now it's on a boot loop! and since buttons not working i can't go to DFU mode! what should i do? it's jailbroken by greenpoison.

1) You should never have erased all content and settings, this is WIDELY POSTED in these forums. You've 99.95% made the situation much worse than it was before (though THANKFULLY, they're still signing 4.3.3 and 4.3.3 is currently jailbreakable with an untethered jailbreak). Erase all content and settings erases the iOS revision and leaves the phone in a bootlooped state until you can complete a restore on the device. The good news is that if you're not relying on an unlock, you can restore to factory 4.3.3 and jailbreak with the latest redsn0w and have an untethered jailbreak. If you need an unlock, you'll need to create a 4.3.3 iOS custom firmware with PwnageTool so that you block the baseband update.
2) I do not believe you can get the iPhone 4 into DFU mode if your sleep button is broken. There were ways on the 3G and 3GS to do so by uploading a corrupt ramdisk, which would force the phone into DFU mode -- but I do not believe the same exists for the iPhone 4. Olethros is a little bit more versed in this topic than I, so he may chime in with some better info.

You *will* need to restore your phone to get it into a working state from here, but I do not believe you need DFU mode unless you are going to be restoring a pwned firmware, in which case you need to use pwned DFU mode anyways, as far as I can recall.