Solely my opinion, but...
Sales numbers indicate lots of people have and will upgrade to the iPhone 6 and 6 Plus. And while the 6 alone initially felt awkward (still does), people already have and will get used to it. So reverting to a smaller screen size will undoubtedly be a 'downgrade'. Apps and web sites are already being reimagined to take advantage of additional screen real estate, so is there any point to bringing back a 4" screen? Sure I loved it; there are times when I miss it, but isn't it a thing of the past now?
Also, in terms of keeping it "simple", how simple does "iPhone 6 Plus S" sound? Getting to be a bit like Mercedes-Benzes strange naming convention.