I am not saying that the Note 4 will outsell the 6+ but I think it is a possibility, simply for these reasons:
A lot of people use the device for productivity and Samsung has had time to develop the device for the kind of use that Productivity users want. Ie: multi windows, S Pen, Snap capture which translates to text etc. So the software is more mature. That and you don't have to deal with the hoops that Apple wants for you to drag and drop etc files (itunes or having to use drop box etc). You don't need work arounds. So even up from the Note 3 there have been solid software works from Samsung to make use easier. I for one love the idea of easily being able to use the S Pen to highlight text and share etc pretty fluidly rather than trying to spread it across the paragraph with my fingers.
Also, although there are lots of users who wanted a bigger screen, it seems that the sweet spot for the new iPhones are the iP6 over the +. It really seems that people have swallowed the line that Steve Jobs fed them so long ago that one handed sized phones are the way to go. Many are even upset that they feel no smaller options have been presented with new tech if you read these forums.
I know not a lot of people use the Stylus, but Samsung has done a lot to make it more functional for people to use on this iteration of the device.
I think it will be a close battle and that is a good thing because Samsung will have to keep upping their game.
oh and personally if there was another manufacturer with less Touch Wiz, offering the kind of stylus support and software features that Samsung does, I would seriously consider it.