1. Francium (Fr): This is the most reactive alkali metal, but it's also extremely rare and radioactive, making it difficult to study.

2. Cesium (Cs): Cesium is highly reactive and reacts violently with water, even at room temperature. It's often used in atomic clocks.

3. Rubidium (Rb): Rubidium is also very reactive and ignites spontaneously in air. It's used in some photocells and lasers.

4. Potassium (K): Potassium is the most common of the four, and while less reactive than cesium and rubidium, it still reacts vigorously with water and air. It's used in fertilizers and other industrial applications.

Remember, reactivity increases as you move down the periodic table within a group. This is because the outermost electron is further from the nucleus and more easily lost, leading to more vigorous reactions.