Which relation is not a function

1)

(X-2)^2 + Y^2 = 4

2)

X^2 + 4X + Y = 4

3)

X+Y = 4

4)

XY = 4

The first one. When X is 2, Y =2 or Y = -2. Which means Y is not a function of X.
When Y = 0, X = 4 or X = 0. Which means X is not a function of Y.
So it can't be a function in any case.

By definition a function f(x): A -> B (where A is the domain and B is the codomain) can only have one value for f(x) in B. Consequently, Equation 1 is not a function since, being the equation of a circle, there will be two values Y for each X.

However, by suitably restricting the codomain to non-negative (or to non-positive values), Equation 1 can be regarded as a valid function.