I may be misunderstanding, but I believe that the proposal was made much longer ago than 2017 and its original duration was closer to 20 years than 4. It became

I may be misunderstanding, but I believe that the proposal was made much longer ago than 2017 and its original duration was closer to 20 years than 4. It became limited to 2017-2020 because the proposal was only accepted (by Pinker) in 2017, having lain unclaimed until then. Hence Pinker's remark about 15 years of hindsight, and the fact that some of the comments below the line were posted 19 years ago. With this in mind, I'd suggest that 2020 was picked as an arbitrary "sci-fi future" date rather than for an arbitrary term of 4 years. Still arbitrary yes, but more understandable in my view.