[Phssthpok]

I have inherited an old Kindle (4th gen, silver with a row of buttons along the bottom of the screen, non-touchscreen). I want to upload some books and give it as a present to someone, but it ignores any AZW3 files I copy across. It does however recognize MOBIs, but I hate MOBI as a format -- it doesn't display nearly as well as AZW3 (e.g. text justification).

I thought it might be a firmware version issue (I read elsewhere that Kindle 4s can handle AZW3, but I wasn't sure whether this was true with all firmware versions and couldn't find any info about that, and I couldn't find the firmware version) so I downloaded 5.1.0 (step 1 of 3 for upgrading to 5.3.7.3), copied it across to the root directory, and restarted. The "update" item on the menu is greyed out, and the bin file is just sitting there.

So I thought I'd double-check the firmware version. I checked how to do it, and was told to look at Settings > Device Info. This has the MAC address, serial number and "Network capability: Wi-fi", but no firmware version. At the bottom right of the screen it says "Kindle 4.0 (1308590058)". I have no idea what the "1308590058" means.

So my question is in several parts:
1) How do I find out my firmware version?
2) What does "1308590058" refer to?
3) What do I need to do to make this device recognize AZW3 files?

Thanks for any help you can give me!