AFAIK 4000KM track does not mean 4000kms of rail route, but total length of rail tracks laid in the section/s, or in other words, track kilometres.
What you are calling as 100KM, is not defined as 100KM in IR vocabulary, but 100RKM or 100 Route km. There can be 400kms of tracks in a 100km section, as it is not clearly defined here, so most probably it is track kilometres here.. Well there is irregularity in Railway data as well, they also consider the total tracks added in old projects in a fiscal year. Hope I explained it in a simple way.