Main Memory= 4GB=2^2 * 2^30= 2^32 Bytes
Physical Address= 32 bits
Cache size= 16KB = 2^4 * 2^10= 2^14 Bytes
Block offset= 64 word=2^6*2^3=2^9 Bytes(because word size is 64 bits)
No. of blocks=2^14/2^9= 2^5
No of sets= 2^5/2^3=2^2
Tag= 32-(2+9)= 21 bit