I'm sorry you are going through this.
To me, your post more reads as asking for advice on how to continue versus advice on how to wean your 4year old. If I misread your purpose, forgive me.
That said, I totally understand the feeling. When mom's are getting negative feelings towards nursing I really encourage them to think about exactly *what* is bothering them. In your case, is it the frequency, the physical sensation of nursing, your DD's actions as she nurses, etc... Think about what is bothering you most and try to lessen/eliminate *that* issue.
For example, many tandem nursers have a real aversion to the feeling of the older nurslings mouth when directly compared to the younger. It just feels "wrong". Making sure that you don't nurse them together/that there is ample time between the nursings helps some women.
I also cut-down on my older nurslings access quite a bit after early days (months, but my older one was only 2.5). It really helped my DD to know that even thought I might be busy during the day, there was always a chance for her to nurse at bedtime.
Good luck!