If you have the sound xing but not the character, here are the 4 characters read this way: 星 (xīng, star; celestial body; spark), 形 (xíng, shape, form, to appear, to compare), 行 (xíng, to walk, to act, a row, a trade), 性 (xìng, nature, character, sex/gender, the -ness/-ity suffix). They split across 3 tones, so the tone is what separates them.