Are the temps to low

The cooler temperatures, especially dropping soil temperature will slow the plant's growth. But, it probably will not kill the plant. Still, slow growth when the days are getting shorter, quickly is not good for yield. Anything you can do to keep the plant and it's soil warm would be good. Since your plants are in pots bring them inside at night if you can. You can also bury the pots in the ground. The larger mass of soil will remain warmer through the night than a pot above ground exposed to the cold air. You can also pile fresh compost around the pot. It will act as insulation against the cold and the decomposing compost will generate heat to warm the soil. If you have electricity a seed warming mat under the pot and wrap the pot in blankets or straw can help keep the soil warm.

Ppl aim for low temps during the later stages of flowering. I really wouldn't bother with trying to put any plastic over it. That'll do more harm than good I reckon, ( could be wrong) But, they'll be fine i reckon. Look up ppl dropping temps during later stages of f for terps and so on.. It is very cold. I've read ppl dropping temps as low as 8c never 4, but that's probably cause they couldn't get it that cold. But again once they get a good dose of sun, they'll be all good. As your plants photosynthesis shuts down when there's no light. So, she's sleeping and there is no growing going on anyways, so what can it hurt right. They are planted, it's going to happen. So, just push on bud, they should be good though.

It's impossible to guarantee they'll be okay, some strains will be more sensitive than others, but your best shot is covering them overnight with something like a plastic tarp. Just make sure to uncover them early in the morning so they get all possible sunlight and moisture doesn't stay trapped in too long. Good luck!