Speed of A = 6 km/hr

=$6\times\dfrac{5}{18}$

=$\dfrac{5}{3}m/s$

Time taken by A = $\dfrac{distance}{speed}$

=$\dfrac{100}{\left(\dfrac{5}{3}\right)}$

=60sec

A gives B a start of 4 m

i.e., B has to cover (100-4)

=96 metre

A beats B by 4 sec

=> Time taken by B to run the race = 60 + 4

= 64 sec

Speed of B =$ \dfrac{Distance}{Time}$

=$\dfrac{96}{64}m/s$

=$\dfrac{96}{64}\times\dfrac{18}{5}$

=5.4 km/hr