Since you shouldn't start breeding until they are at least 24-30 months of age I would think that 4 years old is within the acceptable window.
If the breeder doesn't agree to allowing a breeding of Gemma perhaps you could adopt a puppy from the same line as Gemma. This would be easier and cheaper for you and the wife and less stressful on Gemma. Birth is always a messy risky business. If the goal is to get the same temperament/look/line as Gemma you may be able to accomplish that without breeding.
If the goal is to have a litter of your own to choose from and maybe keep multiple puppies then definitely talk with the breeder. One common requirement for breeders would be allowing them first pick of litter or she may require more money (if breeding wasn't part of your initial contract) or choice of the sire.
I have never bred dogs but I have bred cats (Bengals specifically) and the whole process can be a beautiful experience. However, it is also a lot of work. It is also very expensive. And even if you do everything exactly right and take every precaution there is still risk. You can still lose the mom. You can still lose the puppies. It isn't a risk free venture. And that isn't to say don't do it. Just meaning go in with eyes wide open and be aware. And definitely stay in close contact with your breeder if possible. Having a mentor when you go off on such a venture is such a valuable thing.
I know a lot of other members here have experience in this and can probably offer more insite than myself.