Right! Good.
So, in the Options Bar at the bottom of the screen, you type "4in" in the Width field, "6in" in the Height field, and "300" in the resolution field.
Then crop your photo. When you commit the crop, it will shrink on screen. Now it's at exactly 6x4 size (at the 300 ppi that Frank asked for) ready for you to save as a jpeg and send to him.