Sounds about right. If we think about the maximum rating an adult person could reach within their lifetime from their current starting point with realistic motivation/training amount etc. (so no 10h a day for 10 years bullshit which works for 1/10000 of the people for economical, motivational and health reasons), then 600-700 is the most common upper bound. 700-800 for some, if they are already quite highly rated and/or somewhat talented and/or well-off for the sport physically. I doubt anyone has gotten to 800+ as an adult if they weren't already a 700+ as a 18 year old.
I chose adults to discuss here, because a hypothetical 4 year old is much harder to gauge as it's not clear at all how much of their potential is capped by genetics and how much by their motivation and amount/quality of practice for the rest of their childhood/teen years.