In a nuclear fusion, we need the nuclei to move around very quickly in order to overcome the energy of repulsion. The first step to this is acquiring the temperature. Due to the pressure of the hydrogen gas, this is achieved by the ideal gas equation PV=nRT. After that, it is the atoms shed their electrons that are a barrier to collisions since they repel each other. Then, the remaining bare nuclei collide with each other freely and a small fraction of them form a new nucleus. The nuclei formed from hydrogen are helium ones; they contain 2 protons and 2 neutrons and they are also called a-particles. They are a radioactive product.
To recap, the correct order is:
3, 4, 2, 1 (first to last)