Jones, the Olympic 100 and 200 metres champion, clocked an unofficial 49.57 for the third leg after a conservative first half and electrifying final 200 metres.
She handed the baton over to La Tasha Colander-Richardson for the final leg 15 metres ahead of second-placed Jamaica.
Colander-Richardson had only to hold to win and although Lorraine Graham strove valiantly to close the gap the deficit was too great.
The defending champions clocked three minutes 22.62 seconds, Jamaica were second in 3:22.25 and Russia took the bronze in 3:23.46.
Jones, her face wreathed in smiles, jogged a victory lap with her team mates Richardson, Jearl Miles-Clark and Monique Hennagan.
Earlier she anchored the U.S. 4x100 metres team to third place behind the Bahamas.
Jones, who announced before the Games she planned to win five gold medals, finished with golds in the 100, 200 and 4x400 relay and bronzes in the long jump and 4x100 relay.