My first thought was to clear off the apps and use it as a music player. However, I don't really want to do that. Apple will eventually stop upgrading firmware for it and I don't want to risk losing the apps I have now and the ability to download the apps in the future. I have a first generation touch and it can't download anything useful from the app store anymore. Luckily, I have Plex on the iPod Touch, so it still has some usefulness.
With the 4s in hand, I started thinking about the fact that I have some older audio devices laying around and I could put them together with the iPhone 4S and make them useful again.
Enter the Pure i20 Dock
In addition to being a dock, it also pulls a digital signal from compatible Apple devices. It then gives you the option to send that signal out digitally to a separate DAC (or receiver with its own DAC) or it can convert the digital signal to 24/192 and send the signal out via its analog outputs.
With the iPhone 4s, I decided to use the Pure's onboard DAC and send the signal to a PA2V2 headphone amp that I bought several years ago to go in my car (it really improved the sound quality of a Kenwood Keg I was using at the time). However, when I got a new car, I switched to an iPod Classic for music and no longer required the PA2V2 in the car. I occasionally used it on vacation with headphones, since it is a great portable headphone amp, but it mainly just sits in my museum of unused tech.
So now I have the 4S docked on the Pure i20, the analog outputs feeding a PA2V2 headphone amp, and a pair of ATH M50 headphones are plugged into the PA2V2. This gives my a TON of audio options:
1. Spotify - I launch Spotify on the 4S and start playing a song, then I start the Spotify app on the iPhone 6+ or iPad (whichever I have in hand). This will allow the other device to be used as a remote control for the 4s docked on the Pure i20 via Spotify Connect.
2. Rdio - This works exactly like Spotify above. However, Rdio currently has more Remote Control features that allow you to even control Rdio running on a computer.
3. Airfoil - Easily the most versatile app. The version I have (which unfortunately isn't available at this time) will allow you to turn the iPhone 4s into an Airplay receiver. You simply launch the app on the iPhone 4s and when you pull up Airplay on another iOS device (or computer) it shows up as an Airplay receiver alongside AppleTVs, Airport Express units, and other Airplay receivers. If you download it today, you will have to install Airplay Speakers and Airfoil on a computer to stream to the computer and then back to the 4s (not sure how this will work with Yosemite). (edit: a free app called AirMusic Player will send audio from iOS to iOS, so I would recommend getting that app now in case Apple pulls it)
4. Obviously, if you just want to control the 4s directly, you can run Plex, Home Sharing, DS Audio and many other apps.
The great thing about keeping it on the Pure i20 is that it is always has a 100 percent charge.
With iOS 8, Apple added a feature called Continuity. While I don't have a cellular plan on the 4s, with Continuity, I can still answer calls on the 4s as long as I have the 6 plus nearby (haven't tested how close they have to be with each other, but having the 4s in the pool and the 6 plus in the kitchen would probably work).
Also, since I can use the 6 plus as a hotspot, I also have internet access without being near wifi. I could see this being useful for visits to the beach. I could put the iPhone 6 plus in a zip locked bag and use the 4s on the beach.
Also, with 64GBs of space, I can put some music on it and still use it as a stand alone iPod.
I also plan to look into apps like "Presence" that will use the actual iPhone camera as a security camera. Right now, the app crashes with iOS 8, but I will look into it when they fix the bugs or see if I can find similar apps.
Obviously, it can run the aquarium, photo, and clock apps that will make it easier on the eyes than a blank screen sitting on the dock.
I am still looking into other uses for it as a second iOS device, but if you have any other ideas, send me an email or leave a comment with your suggestions.