How do you beat skyscraper in sniper assassin 5

You can zoom in (Z) and zoom out (X) to see the 4 targets. All are holding rifles, but don't mistake a mop or broom for a gun.

The locations of the 4 :
1) Lower left, first window
2) Upper left, in the far green building
3) On the second rooftop at left center (dark)
4) In the right building, second floor from the top, third window from the right side