Firstly, your breasts are never empty. As a baby sucks, more milk is being made, so there's always some going on!
The fact you're sore - have you had your latch checked? Its always worth it, if you're at all sore. This could be at a peer supporter group, or a breastfeeding drop in, or with a breastfeeding HV specialist, etc.
Orange juice and iron tablets at the same time for the cold and anemia.
I know its tough going, and four months tends to be a slap in the face for rather a lot of mums - everyone talks about how it magically gets easier at three / four months, so you kind of cling to that, and if it doesn't its hard.
You could try giving formula, but it might not make any difference. Also I would say that if you are concerned about your supply for whatever reason, not feeding at night is not the best of plans, as its when your milk is most stimulated.
Have you thought about maybe co-sleeping on a temporary basis?
Sorry its hard for you at the moment.