upperdeck.com Game Rules: When you draw or add this card to your inventory, if you have no other cards in your inventory, you may play it. Look at the top 4 cards of your opponent's deck. If those cards are all the same type, you may void them and draw 4 cards. Otherwise, you may send any number of them to their discard and return the remaining to the top or bottom of their deck in any order.
upperdeck.com Updates/FAQ: In order to void the cards and draw 4 card, the controller of Master Vex must reveal the cards and confirm with their opponent that they are all the same type. The "otherwise" clause refers to both circumstances of choosing not to void them or their not being the same type.
Other official rulings and speculation:
Upperdeck.com link: https://my.upperdeck.com/public/neopets/cards/757
Wayback Machine link: (Updated: 0/0/20XX)