We recommend that, when you are making video files which will be edited again (like your VOB3 and VOB4), you create AVI files. The AVI format is well-suited for editing. If you split your DVD file into pieces and save them as AVI, you should have no problems joining them.
However, the technique that will produce the best possible quality is to simply edit the original VOB file, using the Split or Split Timeline commands to cut out the sections you want to keep and moving them around on the Timeline. Each time you Make Movie you are recompressing the video, and each recompression reduces the quality of the audio and picture.
