Form and rules
How it works
Pretty and fairly usually mean moderately/quite and are common in informal neutral descriptions.
Rather can mean more than expected and often carries surprise, criticism or a marked reaction.
Quite has several strengths: with gradable adjectives it often means fairly; with non-gradable/absolute adjectives it can mean completely.
Quite can come before a/an + adjective + noun: quite a difficult problem.
Degree meaning varies by context and accent, so teach the core patterns rather than treating these words as exact numerical scales.