Are you sure she has no internal fat build up, that goesn't really show? When there are excess cecotropes normally it's because the food is too rich. 4 years is still a good age, I retired 2 breeding does this year, 8 and 6, and just because I have more than enough offspring from the 2 young ones that need to be bred at least once a year.
I try to give mine a lot of exercise (outdoor time including digging tunnels), and a really lean diet before breeding them. Also, a lot of light for a week is said to help to get them in mood - which makes success more likely.