Yea horses for courses ...

A 4.0 will hardly rev over 5000 rpm...
I'm not saying an equal length won't make power BUT with six pipes as in 4.0 they generate SO much underbonnet heat its not funny...
Keep in mind there are log style flo better than others as well..I prefere the one's that have 2" or so collector and pimery pipes from front and back two enter the ends of this 2" pipe and the centres enter direect to turbo flange..These can be looped to make close as equal length as possible...
Looking at some std exhaust manifolds asthetically they look much to be desired!! Thinking Nissan VG30t etc...On 4.0 it doesn't pay to go too big on primeries either.. 1 1/4 works quite well...

Steady the Nispro part wasn't dirrected to you ...
The 4.0 is so big and lazzy even a bad designed [asthetically] manifold makes power...