Apple fixed the PDF exploit with iOS 4.3.4 which was used in the cable free jailbreak method : jailbreakme.com.
What you have to do? :

Head over to TinyUmbrella  immediately and save your SHSH files. If you haven’t heard of  TinyUmbrella before, all you have to do is download the corresponding software for your PC/Mac and plug your iPhone and ‘save’ your SHSH files, simple!  This is highly recommended for all iPhone 3GS, iPhone 4 owners.

(Or)

Launch Cydia on your iPhone and make sure you have your SHSH files saved over there [You will …

iPhone 4, despite its signal woes is a beast when it comes to processing power and pure, raw speed (a lil’ better than 3GS). Jailbreaking iPhone 4 will give you access to themes, productive tools, unlock and more. I tested this approach with a 3GS, 3G and iPhone 4. It is a multi-iPhone jailbreak and requires no computer access! It is the same as described here but I wanted to document Cydia apps for iPhone 4.
Initially, I was unsure if Cydia is even needed for iPhone 4 with multitasking, wallpaper …