It is wrong to compare number of characters in English to number of words in Chinese. The proper comparison would be words in English to words in Chinese. By 4th grade I knew enough English to proofread ycombinator posts. Of course, that is a low bar...

And I think you're off by an order of magnitude about vocabulary size. Chinese vocabulary size around 6th grade is more like 2K - 4k words tops, not 20K. See

https://www.guavarama.com/2015/02/06/chinese-characters-by-g...