Answer
The man acquires a speed of $~~3.0~mm/s$
Work Step by Step
The man's momentum will be equal in magnitude to the momentum of the stone.
We can find the man's speed:
$m_m~v_m = m_s~v_s$
$v_m = \frac{m_s~v_s}{m_m}$
$v_m = \frac{(0.068~kg)(4.0~m/s)}{91~kg}$
$v_m = 0.0030~m/s$
$v_m = 3.0~mm/s$
The man acquires a speed of $~~3.0~mm/s$.
