had a portion of 4000,as it says in chapter 7;

....... and their mother's fortune, though ample for her situation in life, could but ill supply the deficiency of his. Her father had been an attorney in Meryton, and had left her four thousand pounds.

but that , in chapter 50( Please allow me,it has little bearing on the outcome of the story- honestly!-and you did menton it first!)

Five thousand pounds was settled by marriage articles on Mrs. Bennet and the children. But in what proportions it should be divided amongst the latter depended on the will of the parents.

Here we can assume that Mr Bennet added one thousand pounds to the total to be paid to the "younger children",and as I pointed out in my post above,the parents ,as was the usual custom,were allowed to decide in what shares the younger children were to divide the five thousand pounds.