How to unlock iPhone 4 if original carrier doesn't provide the service?

As far as I know, you can't unlock with Apple after you've purchased an iPhone.

In the US, the laws regarding cellphone unlocking haven't changed, but a Library of Congress exemption to the law has recently not ben renewed so you might only legally unlock if your carrier allows.

The carrier lock is imposed by... the carrier. I don't think Apple will ever remotely unlock devices, so you must look for other ways to do it. If your phone is from AT&T and have more than one year and the customer is in good standing, you can request the carrier unlock for the device.

Apple's software clearly consults the carrier database for IMEI to determine which devices should be carrier-locked and which have SIM slots that are baseband unlocked, but Apple doesn't actually input devices into this database (unless you consider them as an agent to the Cell companies when you buy a locked or unlocked phone from Apple and initiate service with your carrier of choice).