Using WindowBuilder for Eclipse 4 Parts is relatively simple, I just have to wrap the whole content of @PostConstruct into a composite and work with WindowBuilder on this composite.
Is it planned to have direct support for Parts in WindowBuilder? I have no idea about the internal structure of WindowBuilder but I think you could treat @PostConstruct similar to the createPartControl() method of ViewPart.
The following would be a simple Part definition without any controls. The names of the methods are flexible with Eclipse 4, I like the existing ones, except that I added a "s" to the old createPartControl method.