Don't forget to check out Whispersync for Voice audiobooks. They also qualify for this offer as long as their regular price is $14.95 or higher.
For example, if you own the Kindle edition of "Hunger Games" you can buy the Audible edition for $3.95. Since its regular price is $24.95, this book will count as one of the four you need to qualify for the $10 offer.
Even the free books qualify if their regular price is $14.95 or higher. If you own the free Kindle edition of "Dracula", you can buy the Audible edition for free. Since its regular price is $29.95, it also counts as a qualifying book for the $10 offer.
Whispersync for Voice
Whispersync for Voice - Free
Edit:
Of the 10 free Whispersync ready Kindle books above, 9 of them have free Audible versions. "A Tale of Two Cities" does not.
The other 9 free ones all have free Audible versions, but only 7 of them qualify for this offer (regular price $14.95 and up). "Wizard of Oz" and "Moll Flanders" have regular prices below $14.95.