Q: I want to unjailbreak my iphone 4

Restore it in Recovery Mode: http://support.apple.com/kb/HT1808.

There is no way to guarantee that you can "unjailbreak" this phone.  Jailbreaking often causes permanent damage that can't be removed by restoring.  In fact, restoring may brick the phone or it may be unstable.

Unauthorized modification of iOS has been a major source of instability, disruption of services, and other issues