I like this trade, without giving up Jaylen Brown. I also don't want to give up Marcus Smart, but think his ceiling is lower than Brown's. I'm a fan of Jimmy Butler, but I think he is becoming overrated. Tell me how good player x, y, z is when they start winning games. Until then, I'm not interested in an overpay. There's a chance Jaylen Brown will be better than Butler.
If Brown and Smart are in the trade, I wouldn't want to include a BKN pick.
Is a core of IT, Fultz/Tatum, Butler, and Horford good enough to beat GS? I'm not so sure.