Here's the skinny. ALL the DVD players you see at wallymart will play back "standard" MPEG 2 encoded DVDs. This answers the second part of your question.
However, some will re-encode video to MPEG4 or DivX and put these files on a DVD recordable media. This is about the same as putting MP3s on CD recordable media. 99.9% of car CD audio players will not play such.
In both cases, you get more content on each disk, but can only play on the Mac/PC/other machine that can play such.
I do not understand the relationship between a dvd recorder/player and the mpeg4 format for storing movies. Can all dvd recorder/players store and read mpeg4 format or only special ones? I would like to record old family movies and wanted to use mpeg4 format to preserve as much as possible. Please explain. Thanks...