2016 version was released in October. Which puts it currently at 2 years and 4 months. So it'll be close to 3 years by the time WWDC rolls around, but well short of 4. But given all the problems they've had, maybe they're pushing up the redesign?
Or maybe they're pushing up the redesign as otherwise there's not much they can do this year due to Intel and AMD being heavily delayed on Ice Lake and Navi.
I already changed what I said but yeah, only 3 years so far. I guess they gonna release 16.5 MacBook Pro ONLY for this year. Intel is a problem but AMD is ready because MacOS Mojave indicates that 4 Navi GPU is already included in MacOS.