You don't have to know all this when somebody releases some report. You interact with people, work, communicate and gain common-sense and knowledge. That common-sense is making me not to believe many reports like this.
Actually the 3-4% is very much in line with most of the BRICS
An estimated 8% of Chinese pay income tax.
An estimated 8% of Brazilians pay income taxes as well.
But these nations have no section of their population completely exempt from taxation. Whereas in our case we have 58% of the total population ( declaring agriculture as the occupation) completely income tax exempted. Extrapolate that factor into the 3.5% and you see that we tax around 8% of the taxable population as well.