It was never 4%. The last figure that I know was 2.8% which I assume is count of tax filings rather than tax payers.
People with taxable income would be at least 10~15 times more. Most auto/taxi drivers have taxable income levels. There are Daily wage workers with taxable incomes. At my work place, the cab drivers that are hired to drive us to work each evades a tax of anywhere from 2 to 5 Lac each per year
Its not just a case of people not making enough to pay taxes. Yes, there many be quite a lot of people who don't have a job at all or make enough money to have taxable income, but there are many who do have taxable income but evade taxes because they are not under the tax radar.