First of all, we need to calculate the work done by the man to increase his speed. The work done is equal to the gain in kinetic energy. The initial kinetic energy is zero, since the initial velocity is zero; the final kinetic energy of the man is
[tex] K=\frac{1}{2}mv^2 =\frac{1}{2} (50 kg)(4 m/s)^2 =400 J [/tex]
so, the work done is equal to 400 J.
Now we can calculate the power, which is the ratio between the work done and the time taken:
[tex] P=\frac{W}{t}=\frac{400 J}{1 s}=400 W [/tex]
