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It sounds as if your 4th ebook's file is missing.
The Sony PRS350 doesn't directly support AZW according to the wiki here on MobileRead. Which implies that calibre is silently and automatically converting the books when you transfer them to your reader (which is exactly what calibre is supposed to do). However, the fact that it cannot convert that 4th book means there is something odd with it, specifically. Which is why I suggest it's missing.
Since 3 of the 4 are going through fine, it is unlikely to be your settings. But just in case, you could always have a look here: Preferences>Conversion>Output.
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