Curse of the Sphinx (Pixiv Fanbox)

Looks like demigod Nico in a pickle when the local museum acquires a new exhibit that takes over the city. If he can't solve the riddle then he's gonna join the others as permanent additions to the sphinx's servants. The riddle goes- Four letters stand to make my name, Should two be taken four remains, And if they should take away one more, My meaning will be as before. what am I?