The new iPad is unlocked and the iPhone 4 uses a micro SIM which is pretty uncommon. But you can take any regular SIM (called properly a mini-SIM) by cutting it down according to the DIY Guide
Next, you’ll have to get a SIM template printed out at 100% (available here). Setting the printer at 100 percent is extremely important, since it will affect the overall size of the future card. And you’ll also need some scotch tape available at hand, plus a mechanical pencil (or even a normal one, but with a very sharp tip), a very sharp knife or cutter, and a pair of scissors (not a very small one, but being able to provide a very clean cut).
If you’ve gotten all the pre-requisites, let’s move on to the actual process.
Print the template at 100 percent size (very important step, that’s why we’ll be reminding you about it);
Place the SIM card over the template and fix it in position with the scotch tape;
Turn the sheet of paper on the other side and you’ll be able to see the sides of the MicroSIM, which we’ll properly mark with the help of the sharp tipped pencil. Be careful to mark it on the inside, where there’s the cut out space;
Using the sharp knife or cutter, trace (forcefully) the template of the future MicroSIM over the marked area. It’s for the better to trace just once, because several attempts might have some undesired results.
Using the scissors, cut out along the lines traces with the knife or cutter. Remove the surplus material and voila! We’ve gotten a MicroSIM card inserted in the adapter. Perfect match.