1. **State the problem:** Calculate the product of 21 and 4. 2. **Formula used:** Multiplication of two numbers is given by $a \times b$. 3. **Apply the formula:** Here, $a=21$ and $b=4$, so we calculate $21 \times 4$. 4. **Intermediate work:** $$21 \times 4 = (20 + 1) \times 4 = 20 \times 4 + 1 \times 4 = 80 + 4$$ 5. **Final answer:** $$21 \times 4 = 84$$ So, the product of 21 and 4 is 84.