Why does Zcash show a circulating supply of only 4 million? On coinmarketcap Zcash is showing 4 million circulating supply: https://coinmarketcap.com/currencies/zcash/

But they say total supply is 21 million. So does that mean only 4 million has been mined so far?
Yes, that's correct. The details can be found here: https://bitcointalk.org/index.php?topic=1664832.msg16714736#msg16714736

And in 2050, only approximately 23% of the total supply will have been mined, according to this