Here are the steps to add a full stop between the fourth and fifth character of a six-character string in Microsoft Excel:

1. Select the cell that contains the text string you want to modify.
2. Click on the formula bar at the top of the screen to activate it.
3. Use the formula
   Code:

   =LEFT(A1,4)&"."&RIGHT(A1,2)

   to insert a full stop between the fourth and fifth character of the text string. (Note: Replace "A1" with the cell reference of the cell containing the text string you want to modify.)
4. Press Enter to apply the formula to the cell.
5. The modified text string with the full stop in the middle should now appear in the cell.

That's it! You have successfully added a full stop between the fourth and fifth character of a six-character string in Microsoft Excel.