4. 3 5/6 - 2 1/2.

$$ 3 \frac{5}{6} - 2 \frac{1}{2} = 3 \frac{5}{6} - 2 \frac{1*3}{2*3} = 3 \frac{5}{6} - 2 \frac{3}{6} = (3-2) + (\frac{5}{6} - \frac{3}{6}) = 1 + \frac{2}{6} = 1 \frac{1}{3} $$

Ответ: $$1 \frac{1}{3} $$