Don't forget this is an extension so the money doesn't kick in till 2017-18. It's a gamble for the Canucks but one with reasonable upside. If Markstrom continues to improve and continues to eat into Miller's PT over the course of this season - by the time the contract kicks in, it could be very good value. They obviously believe he's on the right trajectory - if you wait until the end of the year, he could very well be in that $4-4.5 million bracket.
This isn't $3.667 million AAV for a backup; it's the Canucks hedging their bets a bit and banking on having a solid starting goalie for the next four years at well under market value, freeing up cash to be spent elsewhere. It could blow-up in their faces, sure - but there is a calculation here and Markstrom has the pedigree, physical tools and positive career trajectory to make it worth their while.