Is it too much for a brother to buy a mobile phone for his sister for four or five thousand yuan?

It's normal for a brother to buy a gift for his sister. Generally speaking, it depends on his income. Do what you can, don't spoil your sister.

The relationship between brother and sister should be particularly good, because they are female compatriots and grew up together. Besides their parents, they are also brothers and sisters, so they are also the closest relatives. Of course, there are also brothers and sisters who have bad relations, probably because of some unpleasant things. At that time, the relationship was not good, but after something unpleasant happened, the relationship between brothers and sisters would return to the previous special relationship.