Obviously you're using something other than bWAR.
2014: 3.7
2015: 5.3
2016: 3.8
2017: 3.0
While it's reasonable to assume some decline due to defense, it seems like stretch to expect that to happen to a significant degree in his age 27 season. I would expect something like last year's performance to be the floor defensively, and the ceiling to be around halfway between that and his peak years. I would think the optimistic prediction for 2018 would be in the 4+ range, and the pessimistic to be around 2.5 or a little below. Ender, like a lesser version of Andruw, isn't primarily dependent upon his speed for his defensive ability. I would think that could slow his rate of defensive decline as well.