In the reaction 4HF + Si → SiF4 + 2H2, the balanced equation indicates that 4 moles of hydrogen fluoride (HF) react with 1 mole of silicon (Si) to produce 1 mole of silicon tetrafluoride (SiF4) and 2 moles of hydrogen gas (H2).
Therefore, the mole ratio of HF to Si in the reaction is 4:1.