how to produce combination of 4 letter character?

[Code Doc]

Using string function is way too slow - it might be faster to convert string to byte array and then swap bytes back and forth.
I'm too lazy to start writing anything but suggest trying that approach.

Rhino, I thought about that too, but I figured it might be too complicated for OP to try and understand. Mid$() is rather fast and there might not be much increase in speed when working with byte arrays because my code does avoid concatenation. Eventually, the bytes have to be converted to strings.

Regardless, I hate connecting several And operands coupled with <> probably just as much as you do. I have a feeling there is a way around this using Select ... Case, but I'm getting lazy in my old age also. Sometimes we need AnHn or Logophobic to come to our rescue.