I don't know if you've made your decision yet, but I had an iPhone for 6 years, switching every 2 years with my upgrade. I DO have a Mac Book AND iPad, so I was worried about losing iMessage and being able to sync to all of my devices. However, there were a lot of issues I had where sometimes my iMessages were going to my iPad or Mac Book but not my phone, and vice versa. I would always find a way to work it out, but iPhones and Apple products aren't always perfect either.
Anyway, I wanted to try the big screen and I honestly felt like I just kept getting the same phone every 2 years, so I made the switch to Note 4 with my husband. I think if you're a longtime iPhone person, the customization options are so vast that it can be a bit overwhelming, but if you really enjoy technology (like I do) it is fun to learn all of the options. You can set your Note 4 up to look almost exactly like your iPhone at first and then branch out into widgets, launchers, etc.
If you're up for a new adventure, GO FOR THE NOTE 4. I truly think you will love it, but even if you don't, you can upgrade in 2 years and get another iPhone. You're not stuck for life. My bet is your'e like me, the iPhone is safe and does work well, but are you really excited to buy the same phone again? I wasn't even excited about my upgrade because I was like great, can't wait to spend $399 on the same thing I have with a slightly different shape this time.
I have had my NOTE 4 for about 4 months now (since the launch) and I keep learning new things I can do with it. My husband bought me the Gear 2 watch for my birthday and I LOVE being able to get notifications and use my pedometer, etc throughout the day. There are seriously so many more options with android, but the Samsung Note 4 is a cut above the rest.
Let us know what you choose. I have found these forums to be so helpful in helping me discover new things about my phone. People are so kind and willing to help each other in the learning process. Good luck!