That will depend on the amount of current You consume.
If its just 50 mA You need the dissipation would be 4 Volt x 50 mA=200mW if it would be 500mA that would make 4Volt x 500mA= 2 watt
Look in the datasheet of the 7805, look for the Rthja and You can calculate the heat it would generate with a certain output current (after You calculate the Watt's dissipated)