Here, we have to find the opposite side of number "3". We have number "3" only in dice 1. From dice 1 and 2 we have no common side, So from dice 1 and 4 we have number "5" and number "6" as common. So from the concept of dice with two common number, 3rd number in each dice will be the opposite of each other. So, here number "3" from dice 1 and number "4" from dice 4 are opposite to each other. Hence, the correct answer is "4".