To solve this problem, we should first understand the concepts of relation and function.
First, we should think of relations and functions as a box, that relates input values with output values.
relations and functions are different from each other because relations allow to relate more than one output to the same input but functions allow to relate only one output to the same input.
In this case, we have the expression y=4. If x is the input, what y=4 means is that to every single value we are assigning the value 4. That is, each value of the input has only one value of the output. This means that y=4 is a function