I think she's a beautiful young lady. I see nothing wrong with a 4 year age gap, especially if Adam is mature for his age. I hope she doesn't have any abortions, because they bring a lot of mental anguish with them. I wish her a long and happy career full of victories, followed by a beautiful post-career life with as many kids as she wants. And I hope they're all left-handed and that she teaches them tennis. (Seriously, as the daughter of a teacher and a an apparently even-keeled person, she'd probably be an excellent tennis teacher herself.)
I wouldn't worry too much about her stopping to have kids. In CZ tens of thousands of young ladies are putting off having kids until they are in their thirties. I don't think she'll be any different. At least 10 Grand Slams, then rugrats. I can see that pretty easily. no worries.