iTunes won't allow me to update from iOS 3.1.3 to iOS 4 on iPhone 3G
So I know that when it comes to an iPhone 3G, it is better to keep it at iOS 3.1.3 however I really want to update it to the latest iOS. Thing is, it won't allow me to. Everytime I would click on "update" it would end up saying that my iPhone isn't eligible for the requested field. Its quite annoying and I have no idea what to do anymore. I've tried restoring my phone and I've looked online EVERYWHERE but nothing comes up so I have decided to go on here and see if anyone can help me out. It would be greatly appreciated as I don't know what else to do.
iPhone 3G, iOS 3.1.3