What are Class 4 Roofing Shingles and are they worth the extra cost?

We often come across misinformation about the term Class 4, or impact rated (IR), asphalt roofing shingles.  Some contractors use the terms as a sales tactic to "upgrade" the roof while charging a significantly higher cost to provide this class of shingles.  For this topic we are focusing solely on laminate architectural asphalt shingles typically used on residential roofs.  So what are Class 4 roofing shingles?