Well, the answer is 6237. The clue was the coeffecients of a polynomial, red for negative, black for positive.
http://www.wolframalpha.com/input/?i=x^4-6x^3-31x^2%2B204x-252%3D0
The formula and solution is there in the link.
The solution of x = {-6,2,3,7} gives us 6237 when entered into a keypad.
It was just a little math riddle that I wanted to get out of my head....
So who's next? If any.....