Guest: any answer other than 4100 is wrong. The catch is that you are adding one thousand, plus forty (1040) and one thousand plus thirty (1030) and one thousand plus 20 (1020) and one thousand plus ten (1010).
People adding the numbers up as they go along will often see the need when adding the last number to get to ten tens, and increase the thosands by one, rather than increasing the hundreds by one.
It is interesting to see that, if a wrong answer is given, that answer is so frequently '5000'

Nigel