When you say it is "low quality" what do you mean? Is it blurry? Low color fidelity? Audio issues? Al of the above?
Some basics:
Make sure you enable "High Quality" and "deinterlace" on your quicktime file before you import it into iDVD. ( Clover-J then select the video track, checkboxes in lower right )
In iDVD make sure the "Best Quality" setting is enabled in
Preferences ->Projects ( this is not an issue if your clip really is only 4 minutes long )
If you can it is vitally important that you reexport your final movie out of shake as DV. Call it voodoo, but unless you want issues to pop up, leave it in DV throughout your entire process.
Also, if you have Shake then why on earth aren't you using Soundtrack Pro for your sound, or the builtin sound tracker/editor for Final Cut pro ( or express for that matter ). Quicktime doesn't set levels at all for audio. You will be much happier in one of these apps.
I won't make any assumptions on what software you have already, but save yourself some major headaches and buy final cut express HD at least!
Hope this helps!
Charles
There is also a great intro article to iDVD that you might want to look at, though it doesn't deal with any quality issues:
http://www.kenstone.net/fcp_homepage/idvd_5_stone.html