High majority toughness and instant death

In my last game I faced with a situation, which is not clearly described in the rulebook.
My Haemunculus ran in a squad of grots, so his toughness was 5 because of majority. Then the squad was shot by a lascannon and the Haemy was the closest model. He took a wound but my opponent argued that the model should be insta-killed, because its own toughness is 4.

Is that really so, that we use majority toughness just for a to wound roll and the instant death rule works as normal? And where could I find the written confirmation of this?

Majority Toughness is used to determine the value needed To Wound as you are firing at a unit rather than individual models within that unit. It does not change the Instant Death threshold for a model, which is always based on that model's own Toughness. This is not specifically written out in the rules but the Majority Toughness section applies only to the To Wound roll. After that you move onto wound allocation, saving throws, instant death etc.

I second the Count.

It's how I'd play it as well. Try thinking of the distinction as the unit takes the hits, but the model takes the saves.

Agree with the Count.

Think of it the other way round; if a Haemonculus has joined a squad of Kabalites or Wyches he is rolling with majority Toughness of 3. If instant death applied on the majority, he'd be removed by a single S6 wound.

So while it would be a hilarious way to get rid of high toughness characters and Eldar Guardian weapons platforms, common sense prevails and a model's own toughness is used for instant death purposes.

Thanks for the replies guys! There are many examples when there is no logic in the Warhammer rules, but here is an irrational exception from this trend.