Are you wanting to grow regular garden lettuce, like loose-leaf or iceberg? I don't know what 4c is, since I use F, but both varieties of lettuce are cool weather plants, so I would imagine that cool or even cold water would work fine.

Hi thanks for your reply.

i just want to grow any edible veg throughout the winter period. Any lettuce or anything else for that matter.

4c is around 39f

I used to grow leaf-lettuce on my windowsill in the house in a south-facing window. Turned out pretty well, too, but I used tap water with a little Miracle Grow. As long as your lettuce gets plenty of light, I think it'll be fine. I like experimenting with things like that. Just keep an eye on the lettuce while its growing, and you should be able to tell if the water is too cold for it.