The ^FO command by itself goes to the top left corner of the field. However, you can add X and Y coordinates to the command. These coordinates are in printer dots, i.e. at 200 dpi, X=200 will be 1" to the right. The top left corner is at X=0, Y=0. The command ^FO by itself is equivalent to ^FO0,0. In your case, Y will still be 0 but you need to figure out the X coordinate of the top right corner. That depends on the label size and printer resolution.
If you want to start in the top right-hand corner, you probably also mean that the label should be right justified. In that case, you can add a 3rd parameter to ^FO. Check that your printer supports this. For right justification that parameter is 1. So, for a right justified label, starting at X coordinate 800 (4" at 200 dpi), the command is:
^FO800,0,1
Note that ^PW sets the label width.
More information about ZPL commands can be found in the ZPL Programming Guide.