You need a micro SIM to use with iPhone 4. If you don’t have a Micro SIM you can convert your Mini SIM to a Micro one by using the SIM cutter.The size of the normal Mini SIM is 25 mm × 15 mm where as the Micro SIM has a size equals to 15 mm × 12 mm. However This tutorial will help you on How to insert SIM card into Apple iPhone 4.
1. If you have the New iPhone, There must be a small pin to eject the SIM card tray like the side image.
2. If not, you can use a paper clip. Just make straight the end of the paper clip.
3. Now locate the SIM tray on the right side of the iPhone. Find that small hole to insert the small pin into it and the SIM tray will come out. It’s just like the emergency eject of the CD drive tray with the help of a small pin. Look at the bellow image to find the SIM tray in iPhone 4
4. Insert the small pin carefully into that little hole as shown in the above image. Push it slightly, until the tray comes out. After the simtray came out a little, you can took it out by your hand.
5. Now put the Micro SIM card into that SIM tray and put it back to the iPhone just in the same side up as you had ejected. See the bellow images for help.
Watch the Video instruction bellow to know how to do this.