If there are 3 more J20s this soon, it would mean that 2017 wasn't the last prototype, it's the first LRIP bird!
That would explain all the bigwig fanfare surrounding its first flight.
But let's not get too excited until we see actual pictures of these other 3 new J20s.
Over on CDF, I think it's been translated as saying that 2018 and 2019 are additional test birds, because the Air Force wants to receive J-20 next year and those two additional prototypes will help to achieve tetst and certification... meaning 2101 may be the first LRIP bird.