I've been debating whether I absolutely have to get a new iPhone. If I do, I'd get the year-old 5s. But for a 32gb model, that's $150 plus a $40 activation fee to renew my 2-yr contract on AT&T, unless I go with their Next plan. I haven't done the math, but it appears that the Next plan is a ripoff, so I'd be paying close to $200 for a subsidized iPhone 5s.
Yesterday, when iOS 8 was released, I decided to install it on my iPad mini. Although the new OS can technically be installed on a 4s, I've read that the animations are choppy, and I already find the iOS 7 animations on my 4s to be slow. I am test-driving the new versions of my favorite productivity apps in iOS 8 to see if I can live without them on my iPhone.
I do most of my "consumption" on the iPad anyway, so saving articles to Evernote or Instapaper would happen there. I use my iPhone mainly for task management and brain-dumping; Launch Center Pro and Drafts are the killer apps for me there.
My conscience is telling me to use my iPhone 4s until it is pretty much nonfunctional. It seems wasteful otherwise. Yes, I can upgrade to a 5s and hand my old phone down as an iPod Touch to a family member, but I'm still spending good money for a new product. And any new product uses up resources and has a labor cost as well.