The off-duty single-car crash happened early Saturday morning when the driver of the Marines' vehicle lost control crashed into a bridge abutment just west of the 605 Freeway.
All four Marines were stationed at Camp Pendleton and coming home from a concert in L.A. at the time of the crash.
All four died at the scene.
"He promised that they would be safe and that he loved me, and he called me his sweetheart. when they told me, my whole world exploded because I lost my other half," said Jacinda Stage to ABC7's sister station KGTV, who is the girlfriend of Joshua Moore, Jr.
Moore died alongside Daniel Nichols, Rodrigo Gomez and Jose Martinez.
4 Camp Pendleton Marines killed in crash on 5 Freeway in Downey, officials confirm
The cause of the crash is under investigation.