azw4 .azw4 file extension is a file format developed by Amazon and is used by applications specifically designed for e-readers. It is mainly found on Kindle e-reader devices, a Kindle software is also available for desktops that allows interpretation and reading of azw4 files. It is used to read e-books normally purchased online. The greatest benefit that comes with using azw4 is that text does not have to stay static, so files transferred from a device with a larger screen to one with a smaller screen are adjusted and one does not have to zoom in order to read.
mobi The .mobi filename extension is an eBook file format developed by French company MobiPocket for their ebook readers for mobile phones and PDAs. The company was later acquired by book reseller and online store company Amazon in 2005, a deal which included the mobi format asset. The mobi file format is mostly associated with the mobipocket software package as well as early versions of Amazon's kindle ebook reader device. The format is considered cross platform given its compatibility with several devices including most smartphones that have compatible readers installed. But although mobi files still exist in circulation, the format was superseded by Amazon's AZW format when support and further development of mobi ceased in 2011.