When I first read about the differences in viewstate in .NET 4, I thought like you that it just changed the behavior of EnableViewstate, but it is a little more complicated than that.
Really to minimize viewstate in .NET 4 you need to set EnableViewstate to true on the page, but then set a new property (on the page) ViewStateMode to ViewStateMode.Disabled
this will turn off viewstate for all controls on the page and it can only be turned back on by setting the ViewStateMode property of the child control to ViewStateMode.Enabled. By default, controls have ViewStateMode.Inherit which is why turning it off at the page turns it off in .NET 4.
I guess they did it this way for backward compatibility to avoid breaking things that were built for .NET 2 or 3.5.
Hope it helps,