Ok, this issue came up in a game last week and I couldn't find anything to refute this rule, so I just went with it...
I hit a unit of Temple Guard with the Plague Furnace. The Temple Guard also had a Toughness 5 Character in it. At the start of combat, the Furnace caused the Temple Guard unit to suffer d6 Toughness Tests or suffer wound with no armor saves.
With the Temple Guard being T4 and the Character being T5, which toughness should those d6 tests be taken at?
Page 10 of the Rule Book does say, "Where a model (or a unit) has more than one value for the same characteristic, as is the case with cavalry, for example, a characteristic test is always taken against the highest of the values."