Download iOS 4.1 ipsw file for jailbreaking your iPhone using cooked firmware. As you already know iOS 4.1 comes with various new feature likeHDR photography , multi tasking and Game Centre. You can update your Iphone 4 3GS 3G on the given links to ipsw files for download.
You can download iOS 4.1 ipsw files directly from Apple servers. Please note that these are large files [400 megabyte and above] which can take some time to download. You can set and forget the download on your iPhone or iPod touch 4 using this free app called Download Lite on Apple Store.
Download Lite is available in Apple Store for free download. You can either download the ipsw files on your computer or your iPhone iPod touch 4. Given are the links below :