Q4 2015? Jeez, it will be time to buy the Note 5 by then.
So you're expecting to a yearly upgrade, I assume? I am just typing my expectations for those who will hold onto a Note 4. I mean the Note 4 is still a viable phone. For me, I think it will come down to the best phablet on the market, aka the Note 5, iPhone 6S Plus, or HTC M9 Phablet. For me, I just want good battery life, timely updates, water resistance, a fingerprint scanner, dual speakers, top of the line camera, great multi tasking, and good build quality. I feel like each of those are dabbling into each phone's perks. Nobody can beat Samsung w/ battery life and multi tasking. HTC has the best speakers, timeliest android skin updates, and good build quality. iPhone has the best support for phones, top of the line camera, and good build quality. S5 had water resistance, albeit, half-baked.
----------
Don't believe that will happen. Yes they eventually update the older devices and at this stage the Note 5 will be around the corner. But old phones tend to get updated to new version of Android - but not always the newer version of Touchwiz UI. Likewise given how long it is taking to roll out Lollipop - Android M won't be on Note 4 till at least Jan/Feb 2016 .....
In your case, Note 5 it is.