Here's why:

* Composition: An alpha particle is essentially a helium nucleus. It consists of two protons and two neutrons.

* Mass: Protons and neutrons each have a mass of approximately 1 amu.

* Calculation: Therefore, the total mass of an alpha particle is approximately 2 amu (protons) + 2 amu (neutrons) = 4 amu.

It's important to note that the actual mass of an alpha particle is slightly less than 4 amu due to the binding energy that holds the nucleus together. However, for most practical purposes, 4 amu is a good approximation.