To be clear infront of you, I have tried the following guide several times until it finally worked thanks to the original Cydia binary and Johnny's Cydia patch. Anyway, you can now patch Cydia on jailbreak iOS 4.2.1 devices with simple steps can be found below.
STEP 1: First of all, you need a jailbroken device running iOS 4.2.1, so you can use our
posted here to jailbreak iOS 4.2.1 on iOS devices using Redsn0w.
STEP 2: Download Cydia.patch file for iOS 4.2.1
from here and copy it to
/Applications/Cydia.app/Cydia_ by
SSH’ing into your iOS device (using the guide
posted here to enable SSH on your iOS device).
STEP 3: Now type the following commands in terminal app on Mac OS X by using the original Cydia binary from here:
apt-get update
apt-get install bsdiff
cd/Applications/Cydia.app/
cp Cydia_ Cydia.bak
bspatch Cydia.bak Cydia_ Cydia.patch
chmod +x+s Cydia_
STEP 4: Finally, reboot your device and enjoy Cydia on your device.
If you rely on unlock, it's recommended you stay away from Redsn0w as you will lose your unlock on your iOS device. From now on, you will need to boot using Redsn0w whenever your device is rebooted until
comex make iOS 4.2.x jailbreak untethered. So, whenever you want to SSH into your device, you must first run your device in the “jailbroken state” on every reboot by using “Just boot tethered right now” option in Redsn0w. We'll be happy to answer your questions.
[via Johnny iPhone]
