The iPad mini 4 looks like a fine upgrade. However when it comes down to it, my iPad retina does what I need it to do, and I think I'm better off waiting for now.
I did upgrade from my original mini to the mini retina the day it was released (after owning the original mini for less than a year). But I use my mini for reading a lot, including pdfs, and the retina screen made it significantly more pleasant, so I'm happy with that decision.
I admit I am tempted though, and I haven't actually gotten to put my hands on one and try it out for myself yet.