4 Kittens Crying in the Rain

I heard 4 kittens crying in the rain in my backyard. I put them in a box and brought them in my living room. I got a bottle for a human baby and kitten formula. I tried feeding them the formula. The grey kitten is not interested in drinking. All it wants to do is sleep and it's not as active as the other kittens. The black kitten drank the most. The two white kittens drank a bit. I'm worried about the grey kitten. I got them out of the box because it was wet. All the kittens are on a dry towel and blanket sleeping right now in my living room.

Tomorrow morning, I'm calling a shelter to pick them up. What should I do until then?