Since you have a 4a which has a headphone jack, you can manage with just an aux cable, which is a cable with a 3.5mm male headphone plug on both ends (like this one).

If you'd prefer to connect using bluetooth instead of plugging in a cable, there are also Bluetooth kits that you can leave in the car (plugged into the aux input and power) and wirelessly connect your phone to when you're in the car.