Card type by the first four digits

Write a function cardType(cardNumber) that finds the card type from the first four digits of a card number.

• cardNumber is a string of exactly 16 digits, without spaces. It can start with 0.
• Add up the first four digits. If the sum is 16, return 'Visa', otherwise return 'Master'.
• Return exactly 'Visa' or 'Master', with a capital first letter.

Why a string: a 16-digit card number does not always fit exactly into a JavaScript number. For example, Number('9999999999999999') gives 10000000000000000.