I think it's too expensive to pay for unlocking my iphone4
T_T

pay to jailbreak ? i think you jailbreak it yourself is better !

if you did read the first post there is currently no unlock for that baseband.

Cydia is an application in the iPhone that is installed once you have jailbroken the iPhone. It's not an app you can install like any of the apps from the apple store.

Just follow some of the post where links to guides for jailbreaking has been posted and you will be fine.

To jailbreak your iPhone yes (I'm sure David will not be happy so I will not explain any further as I don't want to get this thread closed).

Let's wait until the JB thread is re-opened tomorrow and I can explain further what you need to do.

Apologies, but I'm respecting the rules of the forum here.

The JB thread is now opened so I will explained further about the DFU mode. There are 2 types of DFU mode, one is the normal DFU mode while another is the limera1n (pwn) DFU mode.

You need to be in the pwned DFU mode to allow for redsn0w to inject the limera1n exploit. Fortunately, redsn0w comes with a great step by step countdown timer for you do put the iPhone in the pwn DFU mode.

Just follow the step and your factory unlocked iPhone should be jailbroken via redsn0w.
For those who have locked set, pwnagetool has been released (for Mac) and this allows you to create a custom firmware which will enable you to preserve your baseband.