Open iTunes. Go to "File" and click on "Add File to Library." Select your MPEG-4 video and wait a second or two for it to import. Go to "Movies." Your movie should be there. You can rename if you want by clicking on "Get Info." Plug in your iPhone to iTunes. Go to the "Video" tab, and make sure that "Sync Movies" is selected, and select your movie. Sync your iPhone. If it doesn't work, or the video will not play, go back to the "Movies" section of iTunes. Right-click the movie, and select "Convert Selection for iPod/iPhone." Wait for it to convert, then sync your iPhone again, making sure that it is still selected under the "Video" tab.
I'm pretty sure it will work the first time, since it is already compatible.