If a fridge set to refrigerate at 4 degrees C, is put outside at -20 degrees, would it heat up the food to 4 degrees?
;j
Yes.
Haj
How? 
Uh, NO. I am making the assumption that when you say “put it outside” you are talking about putting the fridge outside. The fridge has no means of heating. It (the fridge) would lose heat to the outside world, turnoff when at 4 degrees and continue to lose heat to the environment until it to was at the same temperature (-20).
The fridge will only cool. Unless you have some new fangled thing with a bi-directional heat pump in it. I certainly have never seen one.
Because that’s the temp the food is exposed to. A refrigerator is very well insulated.
If a refrigerator is so well insulated, try unplugging yours before going to work in the morning and then when you get home see how well it retains the temperature inside. 
I concur; I cannot imagine any fridge manufacturer finding it a pressing need to incorporate heating apparatus into their products.
Minor hijack: My parents, who have retired but not yet moved to Florida from Chicagoland :D, have a refrigerator in their garage. Handy for pool parties & barbecues, as it’s right next to the door from the garage to the back yard. Anyway, they were advised once upon a time to unplug it in the fall & leave it that way until the spring. Something about damage to the unit if it was trying to run during the cold winter nights. (The garage is unheated, obviously, or this wouldn’t be an issue.) Anybody know why this would be? (Or was someone just talking about something of which they were entirely ignorant?)
I think it must be ignorance. We have had a freezer in our garage for about 10 years with winter temps below freezing. It’s still running great.