Acceleration = (change in speed) / (time for the change)
Change in speed = (speed at the end) - (speed at the beginning)
Change in speed = (10 mph) - (50 mph) = -40 mph
Acceleration = (-40 mi/hr) / (4 sec)
Acceleration = -10 mi/hr-sec
To get a more comfortable, familiar unit for the answer . . .
Acceleration =
(-10 mile/hour-second) x (1,609.3 meter/mile) x (1 hour / 3,600 sec)
Acceleration = (-10 x 1,609.3 / 3,600) (mile-meter-hour / hour-sec-mile-sec)
Acceleration = -4.47 m/s²