What's the difference between m, n, ng and ng in Japanese?

There is a difference. From the phonetics point of view, there should be a front nasal sound, a back nasal sound and a double lip sound. Generally, teaching in a university requires only the first three. This is actually not quite right, because the words of Japanese announcers need to be strictly mastered, but some Japanese people like to make a front nasal sound, usually a rear nasal sound. Pronasal sounds are pronounced in front and back.