How to generate serialized alphebet in VB.NET or C#

You might make a class for this, providing a next method (similar to what Random provides). You might even go recursive.

class Seq
  {
    char[] c;
    public Seq()
    {
      c = new char[] { 'A', 'A', 'A', 'A' };
    }
    public string next()
    {
      string result = new string(c);
      go_next(c.Count() -1 );
      return result;
    }

    private void go_next(int index)
    {
      if (index < 0) return;
      if (c[index] < 'Z')
        ++c[index];
      else
      {
        c[index] = 'A';
        go_next(index - 1);
      }
    }
  }

I would prefer another way as OriginalGriff :
Your Source is a String which contains the 4 Chars.
Now you could get the ASCII-Code from each Char of this String.
'A' hast the ASCII-Code 65 (thats it's Byte-Value) and 'Z' has the Code 90.
Now you increase the Byte-Value of the lowest Char - if it becomes a value greater 90 you set it back to 65 and increase the next Char-Value. Here you have to do the same check (and so on with the other Chars).
At the end you rebuild the String from the 4 Char-Codes again.

ASC gives you the Char-Code
CHR creates the Char from a Char-Code

Quote:

Until now, I have no clue how should start

Looks like you need to learn programming, because even with very basic knowledge, you should be able to get started.

- when something have similarities from the outside, chances are that it also work similarly from the inside.

Quote:

AAAA,AAAB,AAAC,AAAD and when it reach AAAZ, it will auto round up to AABA and continue the incremental.

This is counting with letters instead of digits.

For Counter = 0 to 27
    MyString= ConvertToString(Counter)
    // do something
Next

In number conversion, modulo allow to extract the unit of a number

MyUnit = MyValue Mod 10
MyRemainder = MyValue \ 10

Repeat 4 times to get 4 digits/letters

Say that 0 is 'A', 1 is 'B' ...
you need to convert each unit to letter.

Now, switch to base 26