Of those, I’ve only been to MP (when it was Bistro) and we didn’t bring the kids. We were there around 9pm and there were no kids in the place. It was nice and quiet. I agree; the earlier the better for kids.
I have tried to dine near a couple who did nothing to stop their baby from wailing through our meal (at another restaurant), and it was quite unpleasant.
I don’t think people expect Disney Sig restaurants to be kid-free, just maybe “screaming-free”. The fact that you are posting here for opinions tells me you won’t be like that couple and will be instead be cognizant of other diners.
It is always possible you will get a stink eye, but so long as you are conscientious about noise level, I think they’ll get over it quickly. Babies and kids are cute, when they’re not running around and/or screaming.
Think about your 4y/o’s needs too, though. Are you also booking character or other kid-friendly meals? Bring things to keep him/her occupied at the Sig’s so you can take your time and enjoy.
And consider your needs as well. Will you enjoy the experience as much if you have to babysit or possibly leave if the kids get bored or cranky? I hate rushing through a really good, expensive meal.
Good luck and remember there are lots of childcare options if you change your mind.