He had only rested for 4 minutes, so the heart rate would not have returned to resting rate of 70 beats/min yet.

However, it would not be have remained at 120 beats/min since he had 4 min of rest.

Notice that the heartrate increases 20 beats/min every 2 mins from 2 to 6 mins. That's a total of 40beats/min increase.

But, the heartrate decreases slower than it increases. So from 6 to 10 min, it wouldn't decrease by so much.

You could put your answer at around 95-105 beats/min