The factors of an expression are those that are multiplied by something else, be it a variable of a terms in parenthesis.
In this expression there are "hidden" factors that you normally don't write because it doest change the results in any way:
The expression is
-4(a+b)
Remember the factor can multiply the variables too, in this case the factor is multiplying a and b:
-4(1a+1b)
In equations the variable written alone, is considered to be multiplied by 1, always.
So the second factor is 1.