If you have four or more cards in hand when Krang enters, his second ability won't trigger at all. If it does trigger, the ability will check again as it tries to resolve. If you have four or more card in hand at that time, the ability won't resolve and none of its effects will happen.

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as affinity for artifacts). The mana value of the spell remains unchanged, no matter what the total cost to cast it was.