where can i find the place where writing the hotkey for sampling? cuz of pressing S its not doing anything at all
OH, I don't know, I have never used a shortcut for the Color Picker...
I know, that you can activate a Color Picker in 4.0 with SHIFT+X, but that only seems to work, by first going to the Editor, where you want to pick the color from and then pressing (and holding) SHIFT+X...
I am sure Dwayne 
dillenbata3 has a better and more complete answer to this.
Yes, in 4.0 Shift+x in 3D Viewport in vertex/texture paint mode and image editor in paint is the sample color. It use to be S.
