My cat gave birth 4week ago, since she as lost alot of weight even before she had her kittens?

She should be eating kitten food - canned or dry is fine. It has more protein and fat to help her produce milk and support her own body as well as the kittens. Nursing/pregnant moms can eat up to 2 or 3 times as much as they normally would. Be sure to offer plenty of quality food. If feeding more doesn't work, it's a good time to check in with your vet to make sure there isn't another reason for the weight loss.