HT1338: OS X: Updating OS X and Mac App Store appsLearn about OS X: Updating OS X and Mac App Store apps
Currently Being ModeratedMay 22, 2013 12:05 PM (in response to ashendp)
You don't. You can only upgrade to the most recent iOS, in this case 6.1.3.
If your iDevice is using a version of iOS lower than 5, you will need to use iTunes on your syncing computer to perform the upgrade. Use the Apple link below as a guide for the upgrade.
Also read the instructions from the section entitled "Update your device using iTunes" at the link below.
Information regarding transferring purchases from your iDevice to iTunes on your syncing computer can be found at the link below.
Currently Being ModeratedMay 22, 2013 12:08 PM (in response to ashendp)
iOS is at 6.1.3 so that is what you can iPhone 4 update to.