iPhone 4 uses a Micro SIM instead of a standard sized SIM and the same Micro SIM is also used in iPad. Micro SIM is not provided by cellular carriers in most countries. However, it is very easy to convert the standard SIM into a Micro SIM by cutting it carefully. You can see images here to learn how to cut the SIM for iPad and iPhone 4.
After making the iPhone 4 compatible SIM, follow the steps below to insert the SIM in your iPhone 4.
Step 1: Take a simple paper clip and make it straight.
Step 2: Find the SIM tray at the right side of iPhone 4.
Step 3: Open the tray using the paper clip as shown in the picture below.
Step 5: Power on your phone.
Tip: If you don’t have the original SIM to activate your iPhone 4 with iTunes, head over to this post to know how to activate the phone using a phonebook SIM.