An odd positive for playing young highly talented NHL-ready players who aren't immediate stars, but have that obvious potential in their toolbox, on the 4th line is they are the star of their line.
Both Suzuki and Slaf, even if they aren't getting 16-17 minutes of being the third wheel or just a part of an ok line, were/are forced by necessity to be the driving player of their line, and they are getting 10-11 minutes of it playing against lower-skilled NHLers. When they become the consistently dominant player over a good stretch, in comes the promotion.