The girl, named locally as Holly Hughes, is now in a stable condition after the crash involving a tanker and a van on the A44 in Powys on Tuesday.
Her mother mother Alison Hind, 28, died in the crash.
Ms Hind's uncle Martin Pugh, 47, and his partner's parents Margaret, 65, and John Kehoe, 72, were also killed.
Holly was flown to the University Hospital of Wales in Cardiff after the collision around 15 miles (24km) from Aberystwyth, near Eisteddfa Gurig, between the villages of Llangurig in Powys and Ponterwyd in Ceredigion.
The family's Ford Focus was travelling east on the A44, the transit van was driving behind it and the tanker was going in the opposite direction.
Local councillor Gareth Morgan described the incident as a "great tragedy" for the community.
He said the road was known to be one of the most dangerous in Wales and said the crash called for a major assessment of its dangers.
A Welsh government spokesperson said it was committed to doing all it could to make Wales' roads as safe as possible.
An 18-month-old girl who was the sole survivor of a crash which killed her mother and three relatives is improving in hospital, police have said.