powers of 4 are like 4^1, 4^2, 4^3 and so on
remember that 4^x means 4 times itself x times for example
4^3=4 times iteslef 3 times=4 times 4 times 4
so the multipules of the powers of 4 are just 4 times of the previous
so we will list
4^1=4
4^2=16
4^3=64
4^4=256
4^5=1024
1024 is already more than 1000 so the list stops and is
1,16,64,256