A drop to 4150 would be a 61.8% retracement of the Aug-Dec rally, which is a typical Fibonacci retracement ratio. I think that downward strength below that level would probably indicate a bear market.
Some Elliott Wave analysts are suggesting that, even if the major indices take out their August lows, this is only the C Wave of a Primary IV correction within the bull market. However, I personally think that is an unlikely scenario given that historically, a 15-20% drop like we had last summer (in isolation) is usually enough to end a correction (ex. 2011, 2010, 1998, 1997) . If you get a drop of that magnitude, then a large retracement, and then break the low (as in 2000-2001 and 2007-2008), it is usually the start of something more serious.