I'd guess that "Methylated" implies that the 4% is methanol. This would mean that Methylated Spirits is the UK name for the common "Denatured Alcohol" sold in the US, which would make lots of sense. Methanol is highly poisonous, quickly causing blindness, hence a small amount of it renders ethanol undrinkable. Also, I've read that denatured alcohol (ethanol) contains around 5% methanol, or a bit less.

Fred