I have a 4/3 Ion wetsuit that I use all winter on the central coast which is a comparable climate. This last weekend which I consider pretty cold I was warm in it. I bought a wetsuit hood (the kind that connects under the chest zip flap) which kept me insanely warm and probably wouldnt get used on anything but the coldest days. For $50 or whatever it was i think its great value. I used to only ride in a Xcel infiniti 3/2 which was comparable to the ion. It was a much higher quality wetsuit though with sealed seams and sealed at wrists and ankles.

I have a thermal rashie which I used under my wetsuit on the colder days, it does help.
Maybe go the 4/3 and then put the rashie underneath when it's super cold?

Colder than that get a Drysuit.. kite in all conditions, all day..

5 or 6 hours in a souwester... 4/5.

5mm or drysuit

Today's rig at Sandringham was 4/3 wetsuit, full thermals underneath, spraytop and wet socks. Lasted all day..