Bluebulb Projects' The Measure of Things presents:
3,900 is about two-fifths the number of Written Characters in the Chinese Language.
In other words, it's 0.4170 times the count of Written Characters in the Chinese Language, and the count of Written Characters in the Chinese Language is 2.3980 times that amount.
(zhōngwén, 中文) (Hànzì, 汉字/漢字; Shuowen Jiezi count; distinct characters)
According to the Han Dynasty dictionary, Shuowen Jiezi (說文解字), there are 9,353 distinct Han characters used to depict the Chinese language. Fluency in written Chinese requires mastery of 4,000 - 5,000 of these characters.