Hm, the low gain I would expect for a 4th place finish, but 67 point loss sounds more like what I'd expect from a 7th or 8th place loss than a 6th. Although, you said you haven't played much, and the number of points gained and lost is higher during your first 50 games (or something to that effect), so perhaps that's why that's going on? The bonus for that might just have little impact on a 4th place finish, and be more noticeable on a 6th place.
I can definitely feel like my getting a weak hero has to do with it sometimes. You get put at a nasty deficit if you open by facing off against, for instance, the Curator and Brann on turns 1 and 2, then face AF Kay on turn 3 when she power spikes, followed by a match against a Rat King who has gotten his hero power buff every turn, all while you're playing as George or Elise. And losing a bunch of health early means that a little bad luck hits you a lot harder later.
Oh? Have they announced something about that somewhere?
If so, they should probably address Brann and Cobalt Guardian too. Because nerfing Lightfang will just mean that Brann becomes the way to do Menagerie, and Cobalt Guardian-based mech compositions will have lost one of their only real competitors.