The family was returning from Agra when a speeding car hit their vehicle just a few kilometres before Jewar toll plaza, police said.
Rajesh Kumar, his wife Neelam, their three-year-old son Ayush and a relative, Subhash, died in the mishap.
All are residents of Samaypur Badli area of Delhi and were returning from Agra after attending a marriage function, police said.
The impact of the speeding car was such that Rajesh's vehicle fell on service lane after breaking barriers of the Expressway.
Hunt is on for the other car and police is going through CCTV footages.
Six persons were killed on Yamuna Expressway in February.
