Elven Blood is presented in full Missions format, so as far as I can tell it would be legal for Season 4 characters. This would mean you can play them before you turn your character into a prime runner for Chicago Season, though PM1 isn't out yet. I think that puts Season 4 at a total of 31 non-convention missions available? Namely 04-00~04-12, Free Taiwan, the five Elven Bloods, four Sprawl Wilds, four Firing Line and four London Falling.