One of the things mentioned in that comment thread is that, even with MA as it is today, some 4th graders can still benefit from it.

I can attest to this.

My son completed the 4th and 5th grade courses. They took about a year in total (5 months plus 7 months IIRC). During that time, I spent maybe 30 mins per week giving him some additional explanation. (At school he was studying Singapore Math 3A and 3B, which don't overlap much with these courses.)

If MA didn't exist, I would instead have my son speed run Beast Academy 4 and 5, to make sure he has no gaps, and then I would then need to tutor him 1:1 using the AoPS Prealgebra book, which is good, but a bit thick and intimidating for a child to self-study.

Honestly I’d do the BA. Later he’ll have memories of silly characters, funny pictures with hidden cultural references, and bad jokes, not just grinding XP. We still refer to Grogg and Captain Kraken.

Oh yeah, we have all the BA books and he's read most of comics in the guide books. He really likes them.

But the exercises in the practice books and BA online aren't as good as the MA exercises. And there's no spaced repetition.

Valid, my younger kid forgets a lot of stuff without that kind of reinforcement.