1) Check how pixels are mapped to inches: Image>Print size (note that due to a differences in definition between screen and printer the image on the screen will usually appear bigger than in print (about 3x, if your print definution is 300DPI)
2) At the bottom if the image display set the units to inches
3) Use the Measure tool (

)
4) Click on your first point
5) Depress Alt-Ctrl and reclick, this will create a pair of guides on that point
6) Click a couple of times anywhere on the image
7) Go back tot the first point (you shouldn't have the "Move" icon, otherwise back to 6))
8) Click again and drag. The bottom of the image window will show the distance from the first point in inches
9) At the proper distance, release the button, depress Alt-Ctrl and reclick, this will create a second pair of guides on that point
You now have two points 4 inches apart, you can use any tool to draw the line.