Johannes Eisele/AFP via Getty ImagesThe S&P 500 could see an additional 10% sell-off if it breaks below a key support level.Katie Stockton of Fairlead Strategies highlighted 4,546 as a key level to watch for the S&P 500.Below that, "downside risk would increase to next major support near 4,200," Stockton said.Sign up here for our daily newsletter, 10 Things Before the Opening Bell.The S&P 500 could sell-off an additional 10% from Friday's close if its breaks below a key support level that is fast approaching, according to Katie Stockton of Fairlead Strategies.Stockton highlighted 4,546 as crucial support that the S&P 500 needs to hold in order to prevent further downside.