[aoner]

Modern heat pumps are still highly efficient at 40F (4C) the coefficient of performance is 4+ at this temperature. We switched from an hybrid set to a full-electric setup (also PV and solar thermal) and are super happy with (live in Amsterdam, so regularly goes below 40F/4C). There's probably something wrong with your neighbors setup.

I'd say practical limits of current models are probably around -20C (-4F)

If you're interested here is an example datasheet[1] of an air to water heat pump (2018) (using Celsius here):

Mitsubishi PUHZ-SHW112YAA Guaranteed operating range (outdoor): Heating: -28 to +21 Domestic hot water: -28 to +35

On page 108 you can find the actual co-efficient of performance (COP) for different outside temperatures and inside water heating temperatures. As an example: at -10 and an water temperature for heating at 25 you still get a COP of 3+. That means that 100% electricity moves 300% of heat into your house.

[1]: https://www.mitsubishi-les.info/database/servicemanual/files...