Barron states that 4.18×10⁸ joules equal 1 kcal, is this correct?
This should never have happened in the scientific community, but:


  The energy values of food are measured by the Calorie, with a capital C. A Calorie is equal to 1,000 calories or 1 kilocalorie (kcal). 


Source: here

It is possible that Barron's got confused between Calorie and calorie. Even so, they still would be off by a factor of 100.

According to Wikipedia:


  The SI unit of energy is the joule, with symbol "J"; one small calorie is now defined as exactly 4.184 J, and one large calorie is 4184 J. However, the two units are still used occasionally in technical work, and the large calorie is still widely used in nutrition.


So you are correct in saying that 1 "small" kilocalorie is 4180 joules. 1 "large" kilocalorie is $\pu{4.18e6 J}$, and maybe Barron's is now suggesting an "extra large" calorie.

More likely it is just a typo.