Will a new D800/600/D4 improve my work?

Probably not for the majority of your work. The D4's AF speed will be noticeably faster and there are more AF options. If you need more resolution then the D600/D800.

But like many of us, we upgrade just to have something new with a few extra features. As a hobbyist, that's half of the fun.

--
Peter
Ontario, Canada