I'm using Professional quality which doesn't use as many GBs as Best Performance and this is the only way I can fit it on under the 4.2GB limit.
That's not how it works. iDVD will compress the movie to fit no matter which encoding option you choose. iDVD is only interested in the total playing time of the project including animated menus. If your movie is less that 60 minutes long you can use Best Performance or High Quality. For 60 - 120 minutes either High Quality or Professional Quality will work. For projects over 120 minutes use a double layered disc. For movies with lots of action/motion Professional Quality would be the best choice as it determines which sections of the move can undergo the most compression and which needs less, those scenes with fast moving items.
If you can import the movie into iMove export it out at 480p. Drag the exported movie into your iDVD menu being careful to avoid any drop zones. If you reformat the movie start over with a new project as there's a bug in iDVD regarding adding edited movies to an existing project with that movie already in it.
Follow this workflow to help assure the best qualty video DVD:
Once you have the project as you want it save it as a disk image via the File ➙ Save as Disk Image menu option. This will separate the encoding process from the burn process.
To check the encoding mount the disk image, launch DVD Player and play it. If it plays OK with DVD Player the encoding is good.
Then burn to disk with Disk Utility or Toast at the slowest speed available (2x-4x) to assure the best burn quality. Always use top quality media: Verbatim, Maxell or Taiyo Yuden DVD-R are the most recommended in these forums.
