Why is season 4 the highest-rated season?
Season 4 is generally regarded as the weakest season, due to not being 100% serialized like the other seasons were, yet it had the highest ratings. That's what it says on Wikipedia.
I'm thinking it was due to airing after Lost, another Abrams show, from January to May 2005 on Wednesday nights. Alias had been booted from its original Sunday night timeslot to make way for Desperate Housewives, which Touchstone Television also produced.