I don't think there's anything 'sad' about keeping a 4-year-old entertained during an adult dinner. There's plently of time to teach him to socialise during meals. Perhaps that's a work in progress at their house. All kids do not develop these skills at the same rate.
OP, I have been the person with a portable DVD on (very quietly) at a dinner for my DD with autism and had other kids be distracted. I would have interpreted your comment as talking about you and your family and what you all do during meals, which is, of course, the ideal.
My Dad made a comment once early on about not thinking that I would be the type of parent to do that, and no, I probably wouldn't be if I didn't have a kid who needed it. Incidentally, she is not 4 yet, and we have trained her out of needing the DVD to eat, which is great, but it was a lot of work, and I'd be completely understanding of someone still doing it.
I completely understand how distracting it is for others, though, and we only ever went out when we thought our presence was more desirable than the DVD was annoying. And that wasn't often!