Straight from the study: https://cgdev.org/publication/three-new-estimates-indias-all...
"Estimating COVID-deaths with statistical confidence may prove elusive. But all estimates suggest that the death toll from the pandemic is likely to be an order of magnitude greater than the official count of 400,000;"
But no one is adding this disclaimer, everyone is just parroting the 4 million number.
Another thing to ponder, if this amount of people had died, do foreigners really think Indian people would have sat silently? There may be underreporting or missing stats, but I doubt if it is a 10x difference.