Someone bet $4 million that stocks will post a big drop by Monday

Randy Risling | Toronto Star | Getty Images

Someone made a $4 million bet that stocks could take a dive by Monday afternoon.

Traders said a big trade in S&P 500 short-term puts came out of the blue Wednesday afternoon, and in order to just break even, the would have to be 1.5 percent lower by Monday.

So far, it's not a winning trade, but the question is why did this investor place this bet?