The maximum frictional force is 0.5*10g N =50N

In order for the box to remain stationary with respect to the van the maximum frictional force must be greater that the force that needed to accelerate the mass at the same rate as the van.

In the first case the needed force needed to accelerate the mass at 4 m/s^2 is 10*4=40N, which is less than the maximum frictional force so the box remains stationary wrt the van.

In the second case the needed force is 10*6=60 N which is more than the limiting frictional force, by 10 N, so the box accelerates backwards wrt the van at 1 m/s^2.

RonL