My understanding is that they are wounds to a model, not to a unit. So if he only has 4 attacks he can only remove 4 models.
^This is the exact reason why I think the power of (CC) Strength D weapons is greatly exaggerated. Unless it has a fuckton of attacks like the Lord of Skulls it depends on the stomp attacks to get itself out of a tarpit. The Strength D weapons are designed to kill MSU which the current meta mostly consists of. It can't handle a big blob of expandable infantry.
The Knights are the least powerfull superheavies in the game and once you get over the initial "omg it's Escalation, it's OP! Nerf plx!" you'll find there's hundreds of ways to deal with them and your opponent suddenly has to try very hard to keep the bloody things alive. If you're playing Chaos I would suggest looking into the new Hellbrute formations; DSing multimelta's with the possibillity of firing twice in a turn? Yes pls!