If the unit being attacked contains models with different Toughness characteristics, then the hits are converted to wounds using the majority Toughness in the unit. If there is an equal number of models with different Toughness characteristics, the owning player chooses which Toughness is used. To continue the example above, the unit of Grots (T2) with Ork Slaver (T4) will suffer rolls to wound using the Grots' meagre Toughness as long as they are in the majority. Once again remember that this is figured at each Initiative step. So, for example, if the Grot unit was whittled down to a single Grot and the Slaver, the attacks at the next Initiative step could be resolved at the Toughness of the Slaver at the option of the owning player - all in all a smart move.
Designer's note: The majority Toughness of the unit is used to prevent the combat bogging down into allocating individual attacks against different models and having to make separate to wound rolls. There are a few units with differing Toughness values and this rule prevents weaker models 'hiding behind' the higher Toughness of an anomalous individual.