Reply To: Add (the same) 2 letters to every instance of 4 digits in a 45 page document

Let me try to explain on behalf of Kai.

The Find What :

stands for Word Boundary i.e it will not search for string that is part of a long string. for example it will not search for 5 digit number or “xyz1234”. In your case the numbers are in group of 4.
\d stands for any Digit
{4} stands for the length of the previous GREP Wildcard which is \d. The 4 digits numbers can also be expressed as \d\d\d\d
stands for Word Boundary

The Change to:

PJ stands for the replacement character as per your choice.
$0 stands for Find Text i.e. replace what is being find through the Find What expression.

Hope that helps.