I have a 2 year old who gets very little screen time. When he does get some, his facial expression looks like that of someone on drugs. He stops paying attention to anything else, he barely answers to anything, he only reacts to complain if an ad interrupts the video, or to move his head or complain if you're between him and the screen.

Intuitively, that doesn't look healthy to me, I don't need to look at data to know that. Especially because it's not a reaction that he has to anything else (no other thing, even things he likes a lot, sucks his attention that much... breastfeeding gets close but even when breastfeeding he's less of a zombie than in front of a screen, he does play with things using his hand, foot, etc.) so it doesn't feel normal. And I find it very sad to see kids who spend hours a day in that state instead of interacting with the world, playing, etc.

At 4 months old it's not so obvious because they interact less in general and don't have a lot of obvious states beyond "crying" and "not crying", but check his reactions to the screen when she gets a little older and you will see.