Some kids are very slow eaters. I would enjoy this time as a family as best you can. I wouldn't compare one 4 year old to another as kids are very different and that is okay.
I have two kids and on some days, it's a slow go for finishing dinner. And other days, they wolf it down. The slower process is probably much better for digestion, ya know?
I wouldn't make a problem out of something like this just because you compare to another child. If you feel you can't sit there, then start cleaning up while he's still eating---- do dishes while he sits at the table eating and you can talk together.
My husband and I sometimes, now that my kids are older, miss the lingering at the dinner table with no cares in the world. Now it is off to this or that or doing homework. On Sundays, we make dinner really long on purpose. Everyone is encouraged to sit together and talk. It's really a social skill to enjoy a meal together.
And the obvious question, does he like his meal? Does he have any issues chewing? I will say that my oldest son had difficulty chewing as he has some motor planning difficulties. he also had texture issues. His sensory issues made eating a harder process for him in the toddler and preschool years.
good luck
Are you all eating at a table in a dining room and no television on or other distractions?
It is surprising that when a television is on, they will be more engrossed with that than concentrating on what they are eating.
Don't give him anything to eat or drink that is too close to his meal, because that could ruin his appetite. Going for a brisk walk or allowing him to play outside in the fresh air may help to make him feel more hungry.
Children all go through growth spurts, and when this happens he will start to eat more. Their appetites do vary, so don't compare him to the little girl.
If he doesn't eat for a while just ask him if he has finished and does he want to eat any more. If he says he has had enough then take the plate away. It is better for his age group to have healthy snacks like a piece of fruit or some yogurt in between having their main meals so that they do not get over hungry.
Are there any other things he is sensitive too? Textures, sounds, light, specific foods? Does he do this at all meals?