No. It's not really a "function" at all, properly speaking, and if we bend the interpretation a little then it's a many-to-one function, since no matter what the input value is, the output is 4.
6
Yes, the function y=4 will be a straight horizontal line, passing through the point y=4.
Yes.
yes
Yes it is a function. In standard form it is y=x-4.
Yes.
Yes.
4
y = 2, 4, 6, 8
Y=sin X is a function because for each value of X, there is exactly one Y value.
Yes, since y changes proportionally with x.
y(x) = (4 - x)/4.