How to i update an iPhone 4 with iOS 4.3.5 offline
There exists direct download links for the restore images that you can download from one computer and use for the restore on your own computer, however you will still need an internet connection for the final part of the installation (iTunes needs to verify with Apple servers).

Once you have the .ipsw file on your computer, hold down ⌘ Command (or control if on a PC) while clicking 'Update' in iTunes, and select the .ipsw file for your iPhone.