We would need to know what kind of Mac you have. If your Mac does not support 4K resolutions, then there is nothing you can do to get your display to show natively and you are stuck with scaled mode.
If your Mac does support 4K resolutions, then we'd need to know how the screen is connected. If you're using some kind of conversion to DVI then no, it does not and will not support 4K.
If you are running the screen in scaled mode, then you are not seeing 4K resolution. In the highest resolution you posted above (2560x1440), you are only getting that resolution and your screen is stretching the image (causing a loss in quality).
The fact that your Mac says that the 1920x1080 resolution is not scaled leads me to believe you are connecting via HDMI. In this case, it's only telling you it's not scaled because it doesn't really know how big the display is. But it's still be scaled at that resolution.
If I interpret what my MacBook does correctly, OS X will always drive the screen with its native resolution. The rest is done via DPI scaling, which works way better than with Windows. I never used Photoshop on Mac though, so I can’t really answer your question. – Daniel B – 2014-08-16T10:32:08.540