Addressing: "How difficult is it to crack an encryption that is based on simple four digits?!"
This is a deeper question than you might expect. And much of the following is over-simplified to save time and explaining background concepts. If you are talking about ios8 the answer is different than if you were talking about ios7 or earlier.
In iOS7 and earlier the phone could be imaged, and decryption of the image could be attempted on a desktop system with all 10,000 possible 4 digit pins more or less instantaneously. Also Apple maintained a separate key they could use to decrypt the device if served a warrant.
In iOS8 decryption must be performed on device as it uses a devise specific number (that cannot be extracted from the device) in addition to your pin for encryption; this also prevents Apple from decrypting the device under warrant so you are protected from various three letter organizations serving baseless warrants as Apple has no more advantage in decrypting your device over anyone else. This requirement to perform decryption on-device significantly slows the rate of attempts. If you have "erase data after 10 failed attempts" enabled, then this further slows the process as an attacker would have to image the phone, attempt 10 pins on device (with exponential backoff) and after that run of 10, restore the device with your image.
I'm not familiar with the software that exists to perform these tasks, so I can't estimate how long it would take to try all 10,000 pins under these circumstances; however it is worthwhile to note that the director of the FBI and the US attorney general are throwing tantrums because of it, so presumably it is enough to make their lives more difficult.