The iPhone 4 manual is available online through Apple's support page, apple.com/support. One needs to navigate to the iPhone section through the link provided on this page and click on Manuals on the next page. The manual can be chosen as per the Operating System.
On the support page, click on the iPhone link. This takes you to the detailed support options. Click on Manuals, found at the bottom-left of the page. On the next page, click on Browse by Product. Enter "iPhone 4" in the search box. A list of manuals are displayed. Select the manual that corresponds with the version of your installed OS.