Regular set definition

Question

I came across the following regular sets definition:

Let Σ be a finite alphabet. Regular sets over Σ are defined recursively as follows:

1. ∅ (i. e. an empty set) is a regular set over Σ,
2. {ε} is a regular set over Σ,
3. {a} is a regular set over Σ for all a ∈ Σ,
4. if P and Q are the regular sets over Σ, then also
   (a) P ∪ Q,
   (b) P.Q,
   (c) P ∗
   are the regular sets over Σ.
5. Nothing else is a regular set.

I cannot understand why is this definition complete. What I gather from points 4. and 5. is that
Q.P and Q * are not a regular set. Why doesn't 4. look like this?

4. if P and Q are the regular sets over Σ, then also
   (a) P ∪ Q,
   (b) P.Q, Q.P
   (c) P ∗, Q *
   are the regular sets over Σ.