I didn't get into all the details of some of the posts so this may already be covered a couple of times. I imagine some of the posts are beyond your level at the moment but no worry, it will all come together and some things above you just won't need to worry about. Tommy is correct. Looking at your image data the first three are at the very same distance which I would have to say is the minimum focus distance for the 300mm (450) zoom length. Your clearer forth image is further away. If you want similar shots with this lens in-focus, use your feet.
Dave is also right. Spend some more time with your camera and his suggested settings. Then make individual changes and see what the effects are.
PS. If/when you manage your photos in raw, download the free ViewNX2 editor from Nikon. Besides doing the best job at editing the data, one of the options is to show you the focus point. Handy when sharpening.