Do you plan to make it into a DVD so you can play in DVD players? Or do you intend to just burn the MP4 as is but shrink to fit 4.5 GB?
If the latter, 4.5GB should be more than enough to fit a high bitrate MP4 at full 1080p. However, if you are converting from MP4 to a standard DVD, keep in mind that DVD is not high-def and is therefore, going to lose "quality" if you're coming from an HD source (720p, 1080p).