To remove the last 4 characters within a cell:

1. Select the cell(s) that you want to remove the last 4 characters from.
2. In the formula bar at the top of the screen, type
   Code:

   =LEFT(A1,LEN(A1)-4)

   where A1 is the cell reference of the first cell you selected.
3. Press Enter on your keyboard.

If you want to remove the file extension from the cell(s) as well:

1. Select the cell(s) that you want to remove the file extension from.
2. In the formula bar at the top of the screen, type
   Code:

   =LEFT(A1,FIND(".",A1)-1)

   where A1 is the cell reference of the first cell you selected.
3. Press Enter on your keyboard.