What temperature do you need a winter wetsuit for?

The thinnest winter wetsuit would be a 4/3 wetsuit I think. I don't really think that 3/2 can be categorised as a winter wetsuit. So... how cold must the water be that you will need at least a 4/3 winter wetsuit?

I'll just copy the temperatures from my other wetsuit post (Which wetsuit do I need):

TEMP: 59F–64F / 15C–18C
A good 3/2 full suit, 4/3 wetsuit is better.

TEMP: 54F–59F / 12C–15C
4/3 wetsuit+ booties, If you surf 54F a lot get a 5/3 wetsuits.

This is aproximately it, for more info check the link in that post that I mentioned before.