Insert a column A, then put a counter in that row (1, 2, 3, 4). Then sort the spreadsheet on column A in descending order.
To expand on DLux' answer:
Instead of manually adding the 1, 2, 3, 4, you can:
Enter 1 on the first row, then a 2 on the cell directly below it
Select both cells
Move your mouse cursor to the bottom right corner of the cell that contains the "2". The cursor should change into a + sign.
Click and drag downwards. Excel will fill in the cells you drag over, and increment the values automatically.
This'll save you some time if you've got a large number of rows to reverse.
