I got my Kindle 4 last Christmas, I got my Kindle Fire HD this Christmas, so I have both
However, I use them completely differently because each has its own advantages for specific functions.
What I Use My Kindle 4 For- Reading Amazon books
- Side-loading non-Amazon books
- Reviewing ARCs
- Sending personal documents to read (fanfic, papers, articles, etc)
Advantages Over Kindle Fire HD- Longer battery life
- Ability to use collections (I have so many books, and from so many different sources, that I need them in different collections to keep track)
- Focused purpose
What I Use My Kindle Fire HD For- Reading comics (Color! Wonderful!)
- Media consumption (Movies, music, apps, quick time-fillers)
- Reading the borrowed Prime novel of the month (my free month is ending on Sunday and I won't renew, but it's easier to read those novels on my Fire)
Advantages Over Kindle 4- Color, color, color
- Quick ability to cross-check something online that I run across when reading
- Easier to buy books on the device
So, I enjoy both of my devices and the functionality both provide me. It's been great expanding my Kindle family. My life changed when I got my Kindle and it's changed again since I got my Fire HD