3 is small enough that you can just brute-force it to show you can't do better than 4 (I'd appreciate an elegant solution, but hey, this works).
Which two are the first pair doesn't matter, so let's say the person said they are made from the same material. Now the second pair can be one ball from pair 1 and one new ball or two new balls.
If we pick two new balls, our friend can tell us they are the same as well. Now for the third pair we can pick one ball from pair 1 and one from pair 2, one from a pair and one not from a pair, or two balls not from a pair. In the first case our friend tells us they're different and we still don't know which pair is iron; in the second case our friend tells us they are the same, and we still don't know if this group of 3 is iron or the minority; and in the third case our friend tells us they are the same and we can't tell which of the three pairs are iron balls. Either way we can't tell which ball is made from iron.
If we pick one ball from pair 1 and one new ball, our friend tells us they are the same. Now we have a group of 3 balls, but we don't know if they are the minority or if they are the majority. In fact, no question we ask will help us determine which one it is; if we ask about two balls not in the group our friend can tell us they are equal, and we still don't know if they are iron or not, or if we ask about a ball in the group and a ball not in the group, our friend tells us they are different and there's still the possibility that the balls not in the group are the 4 iron balls.