Question 37466
The word "linear" means a straight line.  Normally any relation or function that is a first power in x is linear. Examples are y = 3x + 8, f(x) = 3/4 x + 2, y = x - 7, etc.
This case, y = -4, is a special case called a constant function...a horizontal line, in fact, without x at all.  
It can be written as y = 0x - 4 to show you that it is also a linear relation.