He's #5 on the team in TOI this year (very close to #4), and #5 in EVTOI (nearly a minute less than de Haan). He gets no PP time, and leads the team in SHTOI.
His TOI numbers suggest that he's being used as a #5, who is their PK specialist.
His actual usage has been up and down the lineup from the 1st pair to the 3rd.
Last year, he was 3rd in TOI and EVTOI, with similar heavy usage on the PK.. The Canes didn't hav De Haan or Hamilton, and they weren't very good defensively.
Personally, as a Leaf fan, my belief is that he's somewhere between a #3 and #4 right now, and his long term upside is that of a high quality #3 (he doesn't possess the offensive game to be a #2).... however, I can certainly see the argument for considering him a true #4 right now. In Carolina, the big minutes have consistently gone to Slavin, and whomever is along for the ride.