The 4*20 displays that I have used are arranged as if they were 2*40 displays that had been chopped in half and the right half glued below. When you write to the display, location 00 is the first character on line 1, location 0x40 (64 decimal) is line 2, 0x14 (20) is line 3 and 0x54 (84) is line 4.
So, if you just write to location 0 it will fill line 1 and then line 3. It will then ignore the next 24 characters (64-40) and then fill line 2 and then line 4.
Nigel, where have you seen a display that has an extra line to switch between them.
Mike.