Jailbreaking an iPhone 4 isn't just legal, it's also easy.
If you're lucky enough to actually own an iPhone 4 and bold enough to break Apple's rules, here's what you do to jailbreak your new iDevice:
1. Open the browser of your iPhone 4
2. Go to JailbreakMe.com
3. Near the bottom of the screen, you will see "Slide to Jailbreak" tab.
4. Pause for a while and ask yourself if you really want to jailbreak your iPhone 4 and void its Apple warranty. If you're 100% sure you wanna do it...
5. Slide the tab
6. Wait for Jailbreak process to finish
7. Enjoy your jailbroken and warranty-free iPhone 4
NOTE: Apple is about to roll-out an iOS update that will prevent users from jailbreaking the iPhone 4 using JailbreakMe 2.0. Install the update once it comes out if you don't like the idea of jailbreaking your Apple smartphone.