Mthakkar, Your thinking about selling 4200 Put and keeping whole premium is attractive trade. But please don't ignore the risk involved with such Naken Short Put.
It is fact that, FEAR is stronger emotion then GREED. If you look at any charts, u will see it clearly that drops take less time.. but recovery takes much longer.
Last year we fell from 4500 to 2500 level between august to november i.e. 3 months and it took nov-June to come back to same level again.
Such events /triggers don't come after telling us in advance. So when market falls, it can fall really fast.. and then your short put may shoot in value. and then u will be forced to buy it back at 3 to 5 times or even higher price. Learn to manage the risk before getting into such risky option trades.
Happy and safe Trading.