They should get along fine. The only concern should be that your girl is arthritic...and when she has had enough, that she get's a chance to lay down quietly without being bugged by the younger dog.
It's true...you can teach and old dog new tricks...you should have no problems training the 4 year old dog.
It will be some work for the first 3-4 months....but since the 8 year old girl is trained...some of her good behavior may rub off on him.
I would get him enrolled in obedience classes. It will help him bond to you, and may show you some new techniques to train him (clicker,equipment,etc.). I would also crate him for the first month or so, till he understand the rules of the house...and your girl has a chance to have him around but not under her feet.
You may also not be able to leave toys or bones around the house like you did before....it may start a conflict, at least at the beginning.
Good-luck!
Gina