Amazon uploaded a new copy of Kindle 4 PC on Saturday (version 1.6, build 327771), and I hope you didn't install it. I did, and much to my dismay I discovered that it doesn't work.
I contacted Amazon immediately, and so far they have done nothing to fix this problem. It's been 3 days that they've let this go on. What's worse is that the borked copy of K4PC is still available for download.
Amazon couldn't give me any useful tips on how to fix it from my end, so I sought help on MobileRead. The fix is pretty simple. All you need to do is uninstall the current copy and reinstall an older version of K4PC.
Don't worry; all your current ebooks should still be there after you reinstall K4PC. They were for me. All of my ebooks and their notes were showed up automatically in K4PC when I ran it.
You can find a copy on CNet's download site that will do the trick. But be careful; I'm told that the install files are country specific. If you download the US version, for example, you won't be able to register it to an account at other Amazon sites, such Amazon.co.uk. I don't know if this is true.
image by burningairlinesgiveyousomuchmore