Which set of numbers does -4 belong to?

Answer 1

It belongs to any set that contains it!

It belongs to {-4},

or {-4, sqrt(2), pi, -3/7},

or all whole numbers between -43 and 53,

or multiples of 2,

or even numbers,

or composite numbers,

or integers,

or rational numbers,

or negative rational numbers,

or real numbers,

or complex numbers,

etc.