No way it takes 3.5-4 years to finish what is a pretty simple project. I liken this to a tad more complex version of the Toon Town -> Storybook Circus change. TT closed Feb 2011 and SC started opening in phases March 2012, finishing in July.
Of course, it won't be that quick. There's more demo to be done and they actually have to buold the Slinky coaster unlike Barnstomer which stood standing with mild thematic changes. However, I don't see how this isn't open for Summer 2018 at the latest (which is perfect, since TS4 comes out then too).