Better? .. IMHO not so much in 4.0 .. Perhaps later (4.1 etc) when they have time to implement some major changes, like re-writing entire workbenck to support 16/24bit screenmodes properly.
But, even though it would not be much different than Os3.9 + number of hacks & patches it's native mode is definitely going to be incompatible.. Ok, you can run well behaving 3.X programs on 4.0 BUT there is no way on running 4.x native softwares on older systems. It's not a biggie now, but someday (if OS4 becomes popular) there will be a lot of new/impover programs that won't work on older machines (including Amitlon etc)..