Ned asked each of his 4 children to think of a 4-digit number.
"Now transfer the last digit to the front and add the new number to the old one.For example, 1234+4123= 5357. Now tell me the results.
The results were told by each of the four children as follows:

Barry: 2348
Maury: 7847
Jaypee: 11847
Derrick: 9846

"Everyone except one is wrong" Ned told the gathering.
Who was it and how did Ned know?