Bookshelf 4 for Kindle Fire requires a Fire (4th Generation) or greater.
How do I know what Kindle Fire Tablet I have
For Kindle Fire tablets manufactured in 2013 and later; Slide down the quick menu and tap Settings then tap Device Options.
In the Device Options Screen scroll to the bottom of the screen and locate the Device Model. This section will tell you what Generation of Kindle Fire you have. Amazon labels their generations based on the year they released the device.
For instance, Amazon released the Fire (5th Generation) in 2015.
VitalSource Supported Kindle Fire Tablets
|Kindle Fire / Year||Model Name||Generation|
|Fire HD 6 (2014)||KFARWI||4th Generation|
|Fire HD 7 (2014)||KFASWI||4th Generation|
|Fire HDX 8.9 WAN (2014)||KFSAWA||4th Generation|
|Fire HDX 8.9 (2014)||KFSAWI||4th Generation|
|Fire HD 10 (2015)||KFTBWI||5th Generation|
|Fire HD 8 (2015)||KFMEWI||5th Generation|
|Fire (2015)||KFFOWI||5th Generation|
|Fire HD 8 (2016)||KFGIWI||6th Generation|
|Fire 7 (2017)||KFAUWI||7th Generation|
|Fire HD 10 (2017)||KFSUWI||7th Generation|
|Fire HD 8 (2017)||KFDOWI||7th Generation|
|Fire HD 8 (2018)||KFKAWI||8th Generation|
|Fire HD 10 (2019)||KFMAWI||9th Generation|