My tomato plants are 4 weeks old. The leaves are curling up and are white.
There are many possible diseases that can attack tomatoes. Leaves turning white could well be powdery mildew: https://www.gardeningknowhow.com/plant-problems/disease/powdery-mildew-homemade-and-organic-remedies.htm
Here's another article that tells about a number of things to watch for: https://www.gardeningknowhow.com/edible/vegetables/tomato/tomato-plant-problems.htm
The only way to be sure what is affecting your plants is to take samples of leaf, stem, and soil to the local Extension Service. This link will help you locate one: https://www.gardeningknowhow.com/extension-search/