Why does the explanation of auxiliary carry flag differ from the article to article? These two images are from Google search.


  AUXILIARY CARRY FLAG: This flag is set to a 1 by the instruction just
  ending if a carry occurred from bit 3 to bit 4 of the A Register
  during the instruction’s execution


Explanation from Quora


  It indicates when a carry or borrow has been generated out of the
  least significant four bits of the accumulator register following the
  execution of an arithmetic instruction. It is primarily used in
  decimal (BCD) arithmetic instructions.


Explanation from Wikipedia

These two different definitions really confuses me.
As I experimented this on Keil using 8051, and it showed that AC is set when the carry occurs out of four low bits. 
What is the precise definition?
The two definitions are exactly the same. Don't forget that in computer science, we often start counting things at zero rather than at one.