Television footage showed burned-out shells of three cars and several brick buildings - mainly shops _ scorched by the huge fireball that burst from the tanker truck on the four-lane highway.
One woman and her young son were burned to death in their car. Another man also died in his vehicle, and a fourth man was declared dead on arrival at a hospital in Toluca, 35 miles west of Mexico City.
Nine people were injured, said a hospital spokesman, Juan Carlos Velazquez, including one man with very severe burns.
The flaming wreckage blocked the highway linking the capital to Toluca.
The truck was carrying two huge cylinders of liquefied natural gas, and its driver was listed as missing.
Tanker trucks regularly ply Mexican roads supplying cooking gas to homes, and several have exploded in recent years, often due to poor maintenance or leaks.
