The RQ-4 Global Hawk is a UAV (Unmanned Aerial Vehicle) in service with the US Armed Forces.The UAV is remote controlled, meaning that the pilot could very well be miles away from the craft as so to reduce the risk of capture in the event of it crashing in enemy territory. It's duties are much akin to the duties of a manned U-2: spying and reconnaissance.
The RQ-4 only appears during Call of Pripyat, where it crashes in Yanov. The RQ-4 is part of the UAV side mission in which the player needs to recover the drone's memory module and bring it either to Nitro or Novikov. It is revealed however that the drone's module only contains data on geographical density and the location of 3 stashes. The three stashes are from Strelok's group and the third one you visit contains his custom shooter.