What is the minimum safe room temperature for puppies?

At 4 months of age a puppy can regulate their own body temperature well enough. Any temperature that's appropriate for an adult dog of that breed is also appropriate for the puppy. That mostly depends on the thickness of the fur and the body size, but even small dogs with thin fur have no problem with temperatures around 20°C.

Puppies cannot regulate their body temperature for the first 4 weeks of their life and may need an additional heat source if the mother is away. After that time, a room temperature between 20°C / 69°F (winter) and 26°C / 78°F (summer) is recommended (Source).

In my personal oppinion the lower (winter) temperature only has to be so high for hairless or very small dogs. Dogs with thicker fur (I'm thinking any sheepdog, retriever or terrier) should be fine with lower temperatures, especially if you give them a warm or insulating place to sleep.