DC DC Boost Converter - Why that high Ampere?
If you assume a converter efficiency of 100% (not possible in reality) the output power equals the input power. 

Output power = 12V * 1.5A = 18W

Input power = Vin Min * I In = 18W

Input Current = 18W / 4V = 4.5A

Under the less than ideal conditions the input current will be higher. The calculator must be assuming some level of efficiency less than 100% which is not directly visible in the information presented in your question.