Most 4/3's aren't that cold in the water. The major issue is when you are out of the water and you get the wind chill. i.e. you fall, get wet, carry on riding. The big thing that improves that is the rubberised outer coating (I think they call it a semi-dry) which effectively stops the wind blowing through the wetty. The con's to the outer coating is that the wetty doesn't last as long because the coating is more brittle.
If you look at the wetsuit pages, you'll notice the rubberised ones generally have a slightly colder rating, around 5 degrees better off.
We used to kite while there was snow on the mountain's. Rubberised ones (from experience) are way better if you're worried about the cold.
This is the rubberised
This only has the rubberised on the chest