A brief math test

This is like the optical illusion - with numbers. You must decide in your head very quickly without using pen, paper or calculator: I have number 1000.
Add plus 40 on it.
Add another 1,000.
Now add 30 Add another 1,000.
Now add plus 20 Add another 1,000. And 10
The sum?
Receive 5,000?
The correct answer is 4100.