I used to use K4PC, but I stopped after a while because when I would change an ebook's metadata or something in Calibre and run it in K4PC again, the program's home menu (and the folder where it stores ebooks) would eventually be filled with older versions of books I wasn't using. Also, Amazon detects K4PC as an extra Kindle, and if I decided to use files sent to K4PC as a starting point in removing DRM from my ebooks, I have to rummage around in the My Kindle Content folder to try and figure out which file is the one I wanted, since K4PC saves them as random strings of characters.
So now, I am instead using
Kindle Previewer to preview Kindle files before uploading them to my K2. Yeah, you have to open the program and then drag-drop the file into it because Kindle Previewer only came out 3 months ago and doesn't have proper file association set up, but I still much prefer it. For one thing, it's much MUCH faster to load and use ebooks on than K4PC is.