I don't have Collins worth this much due to his contract.
Looking at reports, it sounds like #4 is not being included in Collins discussions, and I'd be surprised if Mitchell is included unless it's an overpay. Mitchell just seems like Brown's type of guy and someone that will be very useful to changing our culture.
I could see McNair offering something like...
Harrison Barnes
Richaun Holmes
#37
for
John Collins
#16
...with Holmes being rerouted to a 3rd team and a late 1st/young asset being sent back to ATL. Gives McNair a young, starting PF (granted, I'm not a fan of the fit) while also picking up another mid 1st.