“...fourth-graders are better off being taught how to add, subtract, multiply and divide!”
I’m sorry, but if they can’t add, subtract, multiply and divide well before the 4th grade they are perilously behind for studying a STEM field as an adult.
My son is in 4th grade. I bought the 6th Grade Saxon (7/6) Math Book and after schooled him while in 3rd grade. He caught on quickly and now takes 7th grade math in a public school. It was a hard sell at the school but they let him take the advanced math class with the 6th graders and he’s doing great.
You are correct and, yes, many are that far behind.
The greatest weakness is often in areas involving fractions, word problems or multi-step problems.
Passing the standardized test is not the same as learning to use math to solve problems.