speaking from personal experience, the math I learned up to grade 4 in the Soviet Union was enough to glide through all of American high school math. And I wasn't in any way an advanced math student in the USSR. And I dont seem to recall any extensive, South Korean style homework.
You are probably really gifted. I don't think this is a typical experience, that a fourth grader in any country who doesn't do much homework knows most of US high school math.
In my Soviet school, we went up to Grade 10 US high school math and I have a clear memory when my old knowledge stopped and I had to actually learn. Up to that point it really did feel like just redoing old classes again.
