I wish I knew. I have asked this question of a number of people on a number of occasions, and I have yet to receive an answer.
I have a 1995 female who laid 100% fertile eggs last year, although she did double clutch to my horror. (Second clutch was infertile.)
I know Kathy Love had a 12 year old still laying eggs last year.
You don't want to start them too young/small, but I have never heard of when to stop them. Having said that, obviously if they started to become geriatric in any way you would naturally not want to put the pressure of egg production on them.
A 4 year old sounds to be prime breeding age.
Skye