ip address X.X.X.X consisted of two main parts [Network part] , [host part]
Network part can be consisted of two parts , fixed part called [network summary address] and tunnable part called [Subnet ID] , will reserved by host IPs .
in your example you provide us with the network summary address 172.16. 64.0/18 and you want to subnet this address into 4 subnets (sub 0 ,1 ,2 ,3 ).
let us present your summary address in other way
as you know in binary if you want to represent up to 3 things you need at lest 2 binary digits (00 ,01,10,11) , simply to represent those 4 subnets we will get the two binary digits we need from the host portion as next
so that the network portion of the IP became 20 digits instead of 18
and the subnets will be as shown