I have no problems with the composition of the first two. The second two I don't find as interesting though. I think silhouette shots should *generally* be a large part of the frame. Not always but generally. I think that shot would be a bit stronger with a bit less of the dark bottom bit and more sky - or if possible zoomed in on the treeline. The fourth shot is good for what it is, it just doesn't have as much subject as the first two images. I may have composed it without the bush to the bottom left as it detracts to the simplicity of the lines of the image while not adding any real foreground interest.
On a technical level after looking on you flickr page and seeing the exif I would have done the following:
Image 1: I would have shot at ISO 100 and f11. There's enough light there to allow you the slower ISO and smaller aperture while still maintaining sharpness in a high enough shutter speed and limiting potential motion blur from wind in the trees/subjects.
Image 2: 1/3000 is a very fast shutter speed. As with image 1 you could have dumped the ISO down to 100 from 800 and again used a smaller aperture for greater depth of field sharpness while, again, still maintaining sharpness in a high enough shutter speed and limiting potential motion blur from wind in the trees.
Image 4: this suffers from the same thing as 1 & 2. ISO is too high, shutter too fast and aperture a bit too open.
Ironically you used the most appropriate settings (imo of course) for capturing image 3, the one which is compositionally weakest. For landscapes of this type you want to use the ISO/shutter/aperture that will give you the best detail. I don't know which lens you used and it is my assumption that it would be sharper at f11 than the 5.6 you used in two of the shots. Maybe f9 is best? Something you should know is where the "sweet spot" for sharpness is for your lens. Sorry if this big response seems overly critical but I shared it because I think they are very good images, you have a good eye. Well done!
Oh yeah, one other thing, ditch the vignette.
Unless that's caused by the lens you're using in which case process it out.