I have a bitch who is coming into season every four months. Vets advice is to mate her sooner rather than later as the stress on her uterus is too much for her  with these frequent seasons. Last season was march and now she is in again. She is only 18 months so i wouldnt mate her now but would consider it on her next one which if she follows this pattern would be November. Someone has told me that many bitches with frequent seasons are infertile so i thought i would come on here and ask experianced breeders what they think. She is fit and healthy otherwise and all appropraite health testing has been done. I dont want to rush in and nuetr her if its not necessary

First is this a GP vet or reproductive specialist?

I have had bitches who started with 4 monthly seasons, but by two were going over 6  months, usually being 6 - 7 months. 

None were spayed before 5 - 7 years of age, none bred from before 2 - 3 years.

Only now do I have two young ones who haven't done this and come in less often at 7 - 8 months.

I assume your counting from first day of one season to first day of next?  Less than 4 month seasons may mean not ovulating in some, or cystic ovaries, but at only 18 months she has barely had time to have more than 3 or 4 seasons.

Frequent seasons "can" be related to poly cystic ovaries which "can" cause fertility issues. As Brainless suggested a consultation with a repro vet would be wise, check hormone levels and perhaps a scan of ovaries.