Reason one is that a move to 5000 would closely mirror the likely move by the US S&P 500 to test its all time high. From current levels, the S&P would have to rally a little over 8% to test its all time high, while the ASX 200 would need to rally around 10% to hit 5000.
Reason two is that the ASX 200 is trending upwards, having recently broken resistance around 4450, and the next higher significant resistance level on the weekly chart is around 5000.
Reason three is that 5000 is just beyond the measured target for the big Cup and Handle pattern breakout on the daily chart shown below. The measured target for this pattern is calculated by adding the height of the Cup to the breakout level. So approximately 4410 + 430 = 4840. Please don't confuse this Cup and Handle with the smaller one I labelled last weekend. The Handle shown on the chart below consists of a (smaller) Cup and Handle, as per the weekend post.