x , y coordination division
There is no such thing as multiplication (or division) of two points in the world of ECC math. This is because the points on an Elliptic Curve form a mathematical Group, rather than a Ring. 

As such, you shouldn't expect the coordinates of 4*G and 2*G to be related by the method you described. The result of 2*G is not the same as multiplying each of G's coordinates by 2. You can verify, however, that 

4*G - 2*G = 2*G


The subtraction here is actually shorthand for a complex, but formulaic, procedure for two ECC points. Note that subtraction is simply addition of the additive inverse in the ECC group. This is very different than the multiplicative inverse in the modular field that your Python method defined.