


EPUB is the default (Conversion format) in My Calibre also
That has nothing to do with
seeing books on any connected device
UNLESS
you have a (search) filter eg
formats:EPUB
That only persists until you switch views (Device<->Library)
If you TRY and send a book to your device
AND
it does not have a supported format in the Library: Calibre asks if you want to Convert (to the first supported on the list)
Each device driver COMES with a list of supported formats selected
You can change the preferred order. I would not recommend enabling a different format without exact knowledge of your devices capability to handle it.
Note: If you were a Calibre with Kindle 2 user years ago, your CONFIGURED device list WILL reflect the capabilities of a K2.
Calibre does NOT change an
existing preference.
Later, plugging in a K4 does not auto change it, enabling the AZW3 format. You need to update your device preferences.
Or is your real question: "How can I see which book I have,
that are already IN a kindle format?"

Use the Tag Browser: Formats Section and make a Plus appear nekt to the formats you want to show (You do need to know which those are. There is no Show Kindle [driver preferences] type books)