The AZW4 is an Amazon Print Replica ebook that Amazon developed for Kindle desktop software and Kindle eReader devices. The AZW4 is not similar to MOBI, AZW or AZW3. This format is actually a wrapper for PDF or MOBI ebooks with DRM added.
AZW4 File format
The AZW4 has been introduced as a new version of Textbooks software which is similar to PDF. An AZW4 eBook contains a complete PDF file inside, or it can be considered as a PDF file with a wrapper called AZW4. Primarily the AZW4 is being used by Kindle eReader. However, other applications may also use the .azw4 file extension. Even malware programs can create .azw4 files. Be cautious with .azw4 files coming from a doubtful source.