C++ Vector initial capacity

Question

I have some code which uses thousands of vectors each vector has only 4 entries, So I want to set the initial size of each vector to 4 so that I can optimize memory usage by

Answer 1

each vector has only 4 entries

Then you're probably better off with a std::array instead.

And in case you need 4 elements because you want a mathematical vector, I suggest a struct:

struct Vector
{
    Foo x, y, z, w;
};