This is a quick howto on getting Cydia to run on 4.2.
Remember, this is not a tutorial on how to jailbreak 4.2 / enable Cydia, this is purely for those who are jailbroken on 4.2, but do not have a working Cydia.
This is not for people who are new to jailbreaking, or those who do not know what SSH or APT are.
This can be confusing, though. You need to have APT and SSH enabled on your device.
1. In a command line prompt, run: apt-get install bsdiff
2. Download this Cydia.patch file. Place it in the Cydia.app directory on your device (SSH)
3. Type the following commands in a command prompt on your device:
cp Cydia_ Cydia.bak
bspatch Cydia.bak Cydia_ Cydia.patch
chmod +x+s Cydia_
Reboot. Cydia should run.