Rule of thumb is 1m2 of collector per 50 litres of domestic hot water.
1 person uses approx 40 litres of hot water per day, so 4 people = (4 x 40) = 160 litre solar storage cylinder required and 3-4m2 of solar panels required.
Thermal stores should be sized ¾ of 100litres per/m2 i.e. 4m2 solar panel = 300 litres capacity.