Police said all the four accused were nabbed from different areas of Delhi Tuesday morning for shooting constable Jagbir.
The two adults have been identified as Santosh and Tunda.
The service revolver of the slain constable was also recovered from their possession.
Jagbir was killed and his colleague sustained serious injuries when they were fired on Monday. The attack took place around 1.30 a.m. in west Delhi's Vijay Vihar when the two policemen were patrolling the area.
Police sources said the four accused were going to commit a robbery when the policemen accosted them.