Interesting mathematical trivia I learnt from my daughter yesterday about a special number called Kaprekar’s constant.
Take any 4 digit number with at least two distinct digits. Make the biggest and the smallest number out of these digits and find the difference between the two. Repeat the same process with the new number i.e. the difference that you just found. In a maximum of 7 iterations you will reach the number 6174, the Kaprekar’s constant!
There is a similar constant for 3 digits which is 495.
read more about it at http://en.wikipedia.org/wiki/6174
Alok Goyal's Puzzles Page 
Did’nt work with 4876.
4876-8764=3888, 3888-8883=4995, 4995-9954=4959, 4959-9954=4995 4995 is again repeated. Not 6174?.
Bhraman, that is not correct. for 4786, the largest number is 8764, but the smallest is 4678, and hence the difference is 4086. It goes as follows from there:
8640-0486 = 8154
8541-1458 = 7083
8703-0378 = 8325
8532-2358 = 6174
voila 🙂