Hi Diana,
I’m very sorry for your loss. There is no way to know exactly why she died unless a Vet did a necropsy. If the male is OK, then it wasn’t a toxin or he would have died also. Birds can be sick and hide the signs of illness. If you don’t pick up on a slight behavior change, or change in droppings or appetite, an illness can kill a bird in a day or two. They can also have internal issues that are not apparent, and then they just die. Since she had a male around her, she may have been trying to lay eggs and got eggbound. There are a lot of other issues with egg laying that can result in a sudden death. Even without a male, female lovebirds often lay eggs. Birds have such a high metabolism that any illness can result in a quick death which seems sudden. I’m not sure why you assume it was depression, but that isn’t the type thing that causes death in a bird. If they lose a mate or loved one, they can grieve, but they don’t die from grief. Lovebirds can have genetic issues, especially if they are a color mutation. Some breeders take shortcuts to get mutation or to get as many chicks as possible and this results in weak genes. In that case, a 4 year old bird could easily be middle aged. I’m not sure how you came to have a brother and sister together – if they were in the same cage – but if someone sold them to you and didn’t warn you against keeping related birds together, they possibly breed related birds. This definitely results in chicks with short lifespans, if they even survive at all. So there are so many possible reasons why she died. The important thing is to give the male plenty of attention and extra treats to help him get over the loss. I would not recommend getting another bird right away, if you planned to get another one. He may or may not accept another bird. And never put a younger bird with a mature bird. If you end up getting another bird, you need to get one around his age if you plan to try to introduce them. And you need to plan to keep them in separate cages to begin with, and possibly always.
Take care,
Brenda