The 4x20 displays have the same (physical) size characters as the 2x16, 2x20 etc. So 2x7cm is definitely wrong.
The 4x20 displays are a bit funny as well, because of the way the controller works, the lines aren't in order. It still acts like a "2 line 40 character" display but is arranged like this:
Line1:Chars1-20
Line2:Chars1-20
Line1:Chars21-40
Line2:Chars21-40
So if you just bang in data with the auto-character increment turned on, everything goes from Line 1 onto Line 3, to Line 2 and finally Line 4