What Is a SIM Card?
A SIM Card, or subscriber identity module card, is the card present in most cell phones that contains data specific to the user of the phone. This data can include the user's phone number, cell phone company, and, on some phones, other data like address book entries.
There are three kinds of SIM Cards: full size, Mini-SIM, and MicroSIM, distinguished by their size. The iPhone 4 uses the MicroSIM format.
Location of iPhone 4 SIM
On the iPhone 4, the SIM Card is located on the right side. In roughly the middle of the phone's side, you'll find a small slot with a round whole. This is the tray that holds the iPhone's microSIM.
The SIM on earlier iPhone models was located on the top of the phone between the headphone jack and sleep/wake button.
iPhone 4 SIM Removal
Removing the SIM from the iPhone 4 is a simple process. While Apple includes a "SIM removal tool" with the iPhone, this is just a cheeky name given to a paperclip and isn't required for SIM removal. While that tool works, any appropriately sized paperclip should work.
To remove the iPhone 4 SIM, simply stick the end of an unfolded paperclip into the round hole in the SIM tray until the tray pops out (as shown in the photo above). You can then slide the tray out with your fingers.
Related: SIM Card Is Not Recognized
iPhone 4 SIM Insertion
To insert the SIM into the iPhone 4 again, ensure that the SIM is seated correctly in the tray--it should be flush with the tray, not sticking up at all. The graphics should be facing up and the electronics facing down. Then slide the tray and SIM back into the SIM slot on the iPhone's right side. It should slide in smoothly and click into place. If this doesn't happen, remove the SIM again, reseat it, and try again.
Does iPhone 4 Have a SIM Lock?
In some countries, especially the U.S., the iPhone 4 has a SIM lock. A SIM lock is a feature that ties the phone to the carrier that sold it to ensure that it works exclusively on that carrier's network. This is done most often when the purchase price of a phone is subsidized by the cell phone company and the company wants to ensure that the users will maintain their subscriber agreement for a defined period of time.
In many countries, though, it is possible to purchase iPhone 4s without the SIM lock, meaning they are able to be used on any compatible cell phone network. These are called unlocked phones. Depending on the country and carrier, you can unlock an iPhone after a certain period of time under contract, for a small fee, or by buying an iPhone at the full retail price (generally US$599-$749, depending on model and carrier).
Can You Convert Other SIM Sizes to Work With iPhone 4?
Yes, you can convert mini-SIM cards to work with the iPhone 4, allowing you bring your existing service and phone number from another phone company to the iPhone. This process requires cutting the mini-SIM down to the size of the microSIM. There are some tools available to ease this process (compare prices on these tools). This is only recommended for a tech savvy and those willing to take the risk of ruining their existing SIM card and rendering it unusable.