Calorie is defined as amount of heat required to raise Temperature of one gram of water by one degree (Celsius).

Since heat is a form of energy, hence it’s required to give 4.185 J of work to 1 gram of water to raise its temperature by 1 Degree Celsius.

If we round off 4.185J to 4J, there will be a change of 0.185J of Work and hence our calculations will go wrong.