plants wont die until tempertures reach 32 degrees the temp water frezzes ive even seen plants last in colder but plants wont die over 32 degrees they might however get seriously wiltedYea that's gonna slow growth to almost nothing. And 40's at night could definitely kill the plants depending on strain. If you can wait 3-4 weeks, I think you'd be a hell of a lot better off. If both of those were 10 degrees higher, you'd be fine, in my opinion.