The four characters are Amitabha, the five tones are Amitabha, and there are five tones to pronounce five Buddha names, which are called four characters and five tones.

But then again, why can't four words sing five sounds? Of course, four words can sing five sounds, and you can also sing many sounds. In many songs, a word can be improved by several tones. But this is not the meaning of the four words and five sounds here.