Metroid Fusion

You unlock the Level Four security Hatched In section 4 (AQA), consult a walkthrough if you do not know how, it is just a part of the mission though, on the way to getting the diffusion missiles

Level Four security hatches are found in sector 4, as rambo said. It is hidden relatively weirdly. After preceding into Sector 4 from Sector 5 after killing Nightmare, you will reach a shaft-y thing I believe. In which head down and left through the door to the glass tube-y thing. The blocks above you in the glass tube-y thing are destroyable, and then you can precede up and right, and then left, and then down. This should bring you there, if I remember correctly. I don't know, though. I don't play fusion much.