How many moles of H2SO4 are in 20 mL of a 4M solution?

Is this consistent with the balanced equation of

H2SO4 + Na2CO3 -> Na2SO4 + CO2 + H2O


A 4M solution means that there are 4 moles of substance per liter (the substance is irrelevant). So it's just a simple unit conversion:

4 moles/L * 20mL * 1e-3 L/mL = 0.08 moles of substance.

The equation for sulfuric acid reacting with sodium carbonate is correct and balanced. What it tells you is that one mole of sulfuric acid reacts with one mole of sodium carbonate to produce one mole of Na2S04, one mole of water, and one mole of carbon dioxide.