Smokey could and did leave the island in season 4
I think he was Ben. Or rather, he was possessing Ben. First of all, Smokey lies. A lot. So why does everyone believe him when he said all he wanted to do was leave the island? We saw that all he was really interested in was killing all the candidates so no one could replace Jacob. I believe his whole "I only want to leave the island" bit was him using phrases from dead people he had inhabited (similar to when he screamed "don't tell me what I can't do!"). Anyway, it dawned on me that Smokey did leave the island at the end of season 4. I think when Ben turned the wheel and left the island through the light, and ended up in Tunisia, he wound up being possessed by smokey in the off-island world. We are given several clues about this. Widmore …