I don't think it is strange or dangerous for a 4yo to have a playdate without a parent along, even in an AP family. I was a VERY shy and cautious child (apparently innate temperament), but:
*My mom and 2 other AP moms w/kids similar ages had a scheme wherein each of them would care for (including nursing) all 3 kids for several hours once a week, starting around 18 months.
*I remember going on more standard "play dates" not just at my best friend's house but with kids I'd just recently gotten to know at preschool, definitely at 4 and possibly 3.
*I started staying overnight at my best friend's house (just for fun, not because my parents were away or something) at 5. I remember getting homesick and crying in the middle of the night the first time, but after that it was not a problem at all.
The second and third things I very much wanted to do. The first I don't remember, but Mom's stories about it sound like I didn't object. Maybe that early experience of being in the care of someone else's parent and seeing that she would take care of me too helped me to feel comfortable with the later situations.
Certainly I agree that if your child resists solo play dates, she shouldn't be forced to go; you should respond to her cues. What I disagree with is the idea that it is "shocking" for a 4yo to be separated from mom for a few hours in the care of another responsible adult. Why is that shocking?
I do think my parents could have been more careful about getting to know other families before dropping us off at their houses. My brother and I each had a friend whose house we weren't allowed to go to AGAIN after a dangerous incident (my friend, when I said I had a headache, climbed up the cabinet and got me some adult Tylenol, unnoticed by her mom who was watching TV in another room the whole time; my brother's friend was allowed to play w/the rifles his family stored on the floor under the couch, which were always unloaded BUT STILL!!!
--nobody was harmed, but these were unacceptable conditions for children) and that could've been avoided by screening the families better. I guess I would not let a preschooler go to a new friend's home for the FIRST time without me along. But the idea of forbidding a 6yo from ever entering anyone's home without mom strikes me as extreme. I'm sure Khrisday has good reasons, but as general policy for 6yo that seems weird to me.
Yes, it makes sense that kids who are accustomed to daycare have an easier time going places without mom. I think that's not bad, just different. It doesn't mean the kids are indiscriminate and won't notice if they're in a bad situation or not getting their needs met.