Every episode of Fringe contains a hidden clue that foreshadows something in the next episode. In the season 4 premier of Fringe "Neither Here Nor There", the words "BRAIN FREEZE" can be seen in graffiti in the train yard where Olivia and Lincoln do battle with the new and improved shapeshifters.

I'm afraid my condition has left me cold to your pleas of mercy.

In the next Fringe episode "One Night In October", the killer John Louis McClennan froze the brains of his victims:

Broyles: "He drills into the back of the skull to access the brain. The tissue was punctured and exposed to localized electrical charges. But that's not what killed them. The official cause of death: Cerebral hypothermia. A chemical agent was used to lower the brain's temperature. They froze from the inside out."

Here is a close of of the graffiti: