Plants use the light on/off cycle to know when to veg and when to flower / fruit. To control this schedule, you'll require a specifically designed timer or contactor that can manage the inflow of power from the mains to your lights.
Be aware, and never exceed the power input rating specified for each timer. ie A contactor with 4 plugs, does not neccesarily mean it is power rated for 4 x 1000w lights! A good quality relay and grow light contactor will give you peace of mind with respect to performance, reliability and safety.