In reply to jonzza:
The cheap ones will scratch more easily, but that shouldn't affect sun safety, just longevity.
Cat 4 means it blocks 92 to 97% of visible light, so some cat 4s will be darker than others, but they'll all be fine safety wise, as long as they provide decent coverage of your eyes.
Decathlon have a slightly cheaper pair than the one you linked (non polarised) as well as good value slightly more expensive polarised ones.