Audi A4 coolant problem topping up coolant but still over heating and more coolant needed

Many cars have a bleeder valve to let out air pocket from the cooling system. Fill the radiator, with the cap off, let it idle at about 1500-2000 rpm's until it reaches normal operating temp. on the gauge, then the thermostat will open allowing the coolant to circulate throughout the engine and radiator. any air will then be gone after about 10 minutes at a fast idle. Top off the coolant, replace cap and recheck the level after a short drive of a few miles and has time to cool.
If there is enough trapped air in the system, you will have no heat. The heater box loves to trap a lot of air in it as its like a tiny radiator itself.