What does the 4 mean in the formula4Na2SO3

4 means there are four molecules of Na₂SO₃.

Explanation:

Molecule:

A molecule is the smaller particle which can exist independently.

It may be monoatomic, diatomic, triatomic etc.

For example,

Hydrogen molecule consist of two atoms H₂

Nitrogen molecule = N₂

It may be combination of atoms of different elements.

HCl, H₂SO₄, Na₂SO₃

HCl = 1 mole of hydrochloric acid or one molecules of hydrochloric acid

H₂SO₄ = 1 mole of sulfuric acid or one molecule of sulfuric acid

Na₂SO₃ = 1 mole of sodium sulfite or one molecule of sodium sulfite.

4Na₂SO₃ = 4 mole of sodium sulfite or four molecule of sodium sulfite.

The coefficient before the formula indicate the total number of molecules.