Try adding the files one at a time and see what the meter does, and watch what video settings will be used to record.
With the first file added, you should see the normal size (after transcoding) using it's own encoding quality (hold mouse over title to check). When you add a second one, if Nero needs to reduce the quality (probably bitrate) for it to fit on the DVD, you will notice the change in quality from above. If it fits without reducing quality, the settings should be the same until they need to be reduced. Perhaps the quality cannot be reduced further using all four videos, which gives the overburn.
You can also play with some of the settings, like audio to see if you can reduce the size. I believe LPCM audio is the largest. If you look at the quality settings as far as bit rate, it will even tell you how much video will fit on the DVD at different bit rates.