If you have the sound yi but not the character, here are the 4 characters read this way: 一 (yī, one; unified; as soon as), 以 (yǐ, by means of; using; because; in order to), 意 (yì, meaning, intention, the wish of the mind), 义 (yì, righteousness, duty, justice). They split across 3 tones, so the tone is what separates them.