They do, you are right. But although "quite", "fairly", "pretty" and "rather" do mean approximately the same, they can add different hues to the sentence.
For instance, "rather" can have the most negative hue of them all. On the other hand, "pretty" definitely has the most informal or colloquial flavour.
I don't necessarily agree with fazulamis about the positive/negative "hues" of these intensifier adverbs, which, like mizinamo, I believe that they are more neutral than polarized, but I would like to say that αρκετά should never be translated into English as "very" (πολύ) but rather only as the following (with the first option being the best, and as such it should be made the primary translation):
rather * quite fairly pretty