I can further clarify (not for you, if you've seen the series but for anyone who hasn't) - there is always the gang banger story and then other storylines - police, dock folk, politicians, school scene, and newsroom, that intersect.

Season 4 is totally disjointed IMO - the gang bangers storyline is better than ever but throwing two former cops into the school system and focusing on the kids was the beginning of the end for the C story.

I wouldn't think it was as bad if it didn't lead to season 5, where both the B story (cops with the fake serial killer) and C story (literally unwatchable reporters) sunk things so far from the series heights.