In a 4 for 1 trade directly with Indy, the Pacers would need to cut and eat over $7 million worth of salary or trade them away before completing the deal. So not only would Indy be giving up the best player, they would have to eat at least $7 million in salary or give up picks to trade those players in salary dumps to teams that have space on their roster and in their salary situation to do the trade.(There are not a lot of teams, if any, that can do this at the moment)
Seems to me Indy would be able to close a deal for Turner that wouldn't require them to dump three current players and either their salaries or draft picks in the process.
I don't think that would be as hard as you make it out to be and Indiana has some players they could easily cut and not miss at all. It also wouldn't be that difficult to find a 3rd team to take someone like Poirier for a pick either.
Something like this.
Boston - Turner
Indiana - Kanter, Langford, G. Williams, 2020 BOS 1st, 2020 ATL 2nd (from BOS), 2020 BKN/NY 2nd (from BOS - the lesser pick between BKN/NY is the one at play, so the BKN one), cash - (the 2020 ATL 2nd ATL traded to BOS, though if it is 55-60 it goes to Philly and can't be in this trade - so it likely will be a piece to the Pacers)
Atlanta - Poirier, A. Johnson, 2020 MIL 1st, cash
In that trade, Indiana only has to cut 1 player and can use the cash from Boston to do it. The Hawks pick up another 1st rounder next year for taking on the 2 small salaries into their cap space.
Not sure that really is enough value for the Pacers for Turner, but I think it is a fairly reasonable trade for all 3 teams, especially if the Pacers like Langford or Williams.