For argument's sake I'll say "more children" is anything 2+....2 is the minimum average for avoiding population decline which is never good for any society. In what world does having 4 kids mean that they will all be academic failures??? You also assume that everyone's student debt (if they have any) is crippling to the point of arresting their ability to start a family. In some cases it is, but if the kids are smart they don't go 200k into debt on a useless degree that pays 40k per year. It happens...you reap what you sow. Also, saying that debt becomes a generational snowball is a sad perspective that I hope isn't true...I hope I'm not the only parent that wants to set my kids up with the tools for financial success.
Well, you sort of have to use statistics to run number like that, so based on people I know, most of the 3+ kids family are not able to afford more than raising their kids to high school then let them survive on their own with student loans. I know most of the 2 kids family can afford to pay for their kids' college, and the only child one would be able to send them to private school, tutoring, help them buy their first home early, etc.
I don't care about what population is "good for society". IMO the number will balance itself out and will pay people according to what make sense. So far based on the low vs high skill jobs' pay we as a society values fewer people with more skills, so there is no incentive for me to reproduce too many children when the "customers" want to automate my children's job away at the lower skill level.
I am sure you are a good parent, and you can raise 4 kids fine. I have 2 kids, will never have more than that, was actually considering only one in the past but I figure I can afford 1 more.
What I can say is, had I gotten 1 more sibling I would have to probably pay off my own student loan, bought a home 10 years later than I would have, and pay 900k more due to home price increase, and I would definitely only be able to afford 1 kid instead of 2. I am sure I would have been fine too but I wouldn't be as successful. The competitive advantage of having more resources to raise fewer children is real, assuming you are making the same choices like picking the same major in college (no deadbeat major), having tutoring in high school, buying your home 5-10 years earlier, living in a smaller home but better school district instead of a larger home in a lesser school district, having more parenting time due to shorter commute, etc.
I am comparing my career to my coworkers and roommates from school, so take that with a grain of salt.