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Your pics seem to lack some sharpness, not sure why especially since you have really fast shutter speeds on them. And other than the first pic, none of them really need to be at f/4 - the latter two would probably have benefitted from being shot somewhere between f/8-f/11.
The "Bridge" pic seems like one that could've used bracketing and waiting for the people to get out of it. Good pic, though I personally would think that would've been better closer-up at a wider-angle focal length.
The waterfall pic is the kind that typically benefits from a slow shutter speed - a polarizer can help with that, along with cutting glare & reflections. I'd suggest shooting waterfalls from a different angle, so the water flows from left to right or vice versa, instead of from directly back to directly front, as there's a visual lack of depth.
I too agree with the contrast & curves suggestion. Also saturation and/or hue.