Baby Drinking Less Milk

Is it OK for my 4 weeks old baby to drink less milk because she's too sleepy? Should I continue to feed or just let her sleep? Usually she's drinking 90ml, but for 2 feedings which she woke up crying for milk, she only took 40ml and went back to sleep. She seems frustrated when I try to give her some more milk. She's on milk formula. Is it OK to just let her sleep?