Finally the update to redsn0w version 0.9.6b4 is out which is now available for download and comes with an ability to install the jail broken iOS 4.2.1 apps without using Cydia. Many of them recently commented that Cydia was not working with quite a few devices as it was having the older version. This is very essential for the developers so that they can get to their applications ready and tested for the 4.2.1 release as current version of Cydia is still not compatible with 4.2.1. The bundle software is around 15 MB in size.
Redsn0w has updates its suite so that it can recognize the 4.2.1GM IPSWs. But you can also jailbreak 4.2.1 with a tethered option with which you may face some problems of not working but cause still unknown, hope that the patch will soon be released. The only exceptions to this are the iPhone3G, non-MC, iPod Touch and old boot ROM iPhone3GS. Redsn0w will jailbreak those untethered. If you are on Mac or on windows then you can download redsnOw from the developer’s site.
With the redsn0w and SSH bundle, jailbreak developers and tinkerers can jailbreak and SSH into their 4.2.1 devices but with condition that they have done a tethered boot by using redsn0w’s “Just boot tethered right now” option. Also note that, the Cydia that has 0.9.6b4 is the same one as in 0.9.6b2 and that’s the primary cause why it doesn’t work on 4.2.1. So its highly recommended that you should not try installing on 4.2.1 and instead use SSH bundle or you can also develop or compile a Cydia on your own if you are a developer and if you are familiar with all apt utilities, then you can make use of apt-get to install almost all programs from the command line but don’t forget to apt-get update first which will refresh all your sources.