You can jailbreak 4.2.1 with a tethered one. Downgrading does not brick your phone/pod/pad. 4.2.1 is useless anyway, it's only meant for iPads. Source is easy to add. Just add cydia.hackulo.us to sources in cydia. Jailbreaking does _not_ damage or brick any device, unless you used the wrong version or are seriously mental. Mobile substrate drains battery and uses RAM. MS is used by SBsettings, Winterboard, anything which makes major changes. Install0us does not use Mobile Substrate therefore it does not crap up your device.