logically if we thought like this -

P1 | P2 | P3 | P4 | P1 (vertical lines denotes Context switches)

then before again P1 will be in running state, there will be 4 Context switches, hence 10-4 = 6 units of time is remaining,

To make full use of it, each process P2, P3 AND P4 should run for 2 units of time. (i.e time quantum = 2)

P1 | P2 | P3 | P4 | P1 (vertical lines denotes Context switches)

then before again P1 will be in running state, there will be 4 Context switches, hence 10-4 = 6 units of time is remaining,

To make full use of it, each process P2, P3 AND P4 should run for 2 units of time. (i.e time quantum = 2)