Hi,
My description will sound a little complicated, but if you follow the steps you'll be able to do this.
To import the contents of a DVD into iMovie you need first to open the DVD package by clicking on it's icon on your desktop. (If the movie opens, close it and click on the icon again.) You will see a screen that looks something like this:
Open the VIDEO_TS folder. (Ignore the AUDIO_TS folder, it is of no significance here.) When you open the VIDEO_TS folder you will get a screen that looks something like this:
Option-Drag (copy, not move) all of the files with the extension .VOB to your desktop. These files contain the movie. Be sure only to copy them, not move them, to your desktop or you will disable your original DVD. You can ignore the files with extensions other than .VOB.
Next, download the free app Handbrake and convert the VOB files to Mp4 files that iMovie can read. (iMovie can't read VOB files.) Once you have converted them to Mp4 files then you can import them into iMovie and do whatever editing you want. Do all of the above for each of the 4 DVD's adding them to the same iMovie project, or creating 4 separate projects. From there you share out your project or projects to Mp4 movies on your desktop. Then you drag those shared movies into the project window of a DVD authoring program like iDVD. From there you can burn a new DVD containing all four of your projects. Use DVD-R discs and burn at 4x speed. If you don't have a DVD burner on your computer, you can buy an external DVD burner, like, for example, those manufactured by Lacie.
Hope this helps.
-- Rich