If the leading coefficient is 1, then only the last number counts.
If the leading coefficient id different from one, then the possible factors are of the form
(ax+b) where a is a factor of the leading coefficient, and b is a factor of the last number (constant term).
Descartes rule tells us that the maximum number of positive roots is equal to the number of sign changes. In the above case, sign only changed once from + to -, so there is only one positive root, and a maximum of three negative roots. Maximum means 2,4,6... can be replaced by complex roots, since complex roots always come in pairs.
For example, in our case, there is one positive root (cannot be replaced by two complex), and one or three real negative roots. Since the question says 4 real roots, we can count on 3 negative (real) roots.