I wouldn't feel that comfortable putting Smith out there to kill penalties or defend leads late in the game, but I see no reason why the 4th line can't be more of a skilled line vs a grinding line - we'll probably have a fair amount of physical/grinding players in our top 9 (Tkachuk, Boldy, Nelson, Leonard, Pinto). There's no reason we couldn't run a Smith - Hayes - Brindley or Eyssimont - Smith - Brindley type of 4th line.
But Brindley is the type of guy who can excel in pretty much any role - he was even used a role player during his first WJC stint. His skating, intelligence, and intensity would make him the easy choice over Kunin if I were setting the lines. If he's not getting regular shifts by the end of the tournament, I think it's a mistake.
But I don't expect Hynes to play Brindley or Smith all that much. He's been pretty allergic giving younger forwards meaningful roles in the NHL. Example - during Hynes' last 2 seasons in NJ (up through Dec 3rd, when he was fired), Jesper Bratt was T-1st on the team with an impressive 2.19 5v5 p/60 (tied with Hall). That's 1st line production, yet his 5v5 deployment was that of a 3rd/4th line player. He was 11th among Devils forwards in avg 5v5 TOI and avg total ice time (>300 mins). He was getting less 5v5 minutes than guys like Kenny Agostino, Stefan Noesen, and Wayne Simmonds DESPITE being the most productive player on the team.