“North Korea will develop a plan by mid-August to launch four intermediate range missiles at the U.S. territory of Guam. ... They will fly 3,356.7 km for 1,065 seconds and hit the waters 30 to 40 km away from Guam.”

This looks like what gunnery experts call "bracketing". Fire 4 "test" missiles, each to land 30-40 km away from Guam, but one to each of north, south, east, and west.  If they can determine exactly where each landed, they can adjust the next shot to be closer to, or be on target.

This would also warn the U.S. that NK missiles are capable of hitting Guam, and if they can fit a nuclear bomb on one, they could cause much damage to the U.S. military, while sparing compatriot Korean civilians from harm.