I don't thimk four championships makes Vettel a great driver. The great drivers learn the characteristics of a car and adapt quickly. Senna winning the 1991 Brazilian GP having run the last several laps with only sixth gear is one legend of such adaptability. In a car that is suboptimal, Vettle is underperforming, pure and simple. it is to early to tell if Riccairdo will achieve greatness, but his ability to out drive Vettel does cast shade over the senior driver. Whether or not Alonzo matches Vettels four championships, he has already established himself as one of the great drivers, for repeatedly taking $hitboxes and driving them for all they are worth.
Personally i'd prefer to see the whole lot of them race 1968 VW Beetles with a stick shift and mismatched tires. I'm certain that would show us something about driver ability.