There really is no reason for the Preds to keep all 4 long term, jmo of course.
Generally curious as what Poile could get for each of:
Ekholm
Josi
Ellis
Subban
Why? If you can manage to retain it, there's never a reason to part with stellar defense when it's under contract and for good value. Nashville is one of the few teams in a position to retain them all. They have plenty of cap space.
Unless they can get a superstar center for one of their top four, when it would definitely make sense (but those situations are rare), I wouldn't consider moving any of them.
The reason is mainly because they are covered in all the other critical areas. They have two good scoring centers (Johansen/Turris), a very good checking center (Bonino), and a star winger (Forsberg). They still have a very good goalie - for now. Upgrade center if you can, but otherwise this team should let it ride.
Assuming they re-sign him, the next time they need to revisit this is when Josi's next contract goes into effect in October 2020 (not as early as the moment he signs it, but when it actually goes into effect). They have two more seasons with all four of them under contract and their entire forward core signed (Fiala/Hartman have one more year, but that's it).
As a take, this reads too much like an "it's not fair, Nashville ought to shake loose one of its star defensemen to make it interesting for the rest of us," rather than a cogent argument for why they should trade one of them.