I can't remember the last time it's behaved like zone 4 around here. That's why I list my zone as 5. Sure, we have cold air every now and then - maybe teens below zero, but not often. I'm sure it's been a decade since we've had 20 below at least where I live in the city.
That may be changing.
I was reading the long-range forecast on the Minnesota Public Radio website (they have a very good reputation for accuracy) and to my dismay there's a pool of very cold air way up in Canada that may - just may - drop this way around the middle of the month. We're talking 20-30 below zero for several days.