Ahh, I knew that Rangeland Grazing class would come in handy some day.
As far as expenses are concerned, I imagine that with a bit of searching, one could find a local rancher or (perhaps even better) a local kid in 4H with some sheep that would be happy to have a place to let them graze, with no cost at all to your daughter in law. In fact, it might be much easier to go that route, as opposed to owning and caring for sheep herself.
Environmentally, it is a great idea, but there are a few concerns. A) Sheep are voracious grazers. If left to their own devices, they could easily graze a four acre lawn to the point of no return. Careful attention would have to paid to how many sheep were allowed onto the lawn, and they would have to be moved around often in order to prevent them from grazing one “patch” down to the roots.
B) Sheep are often left to graze in areas that are considered unsuitable for other animals, such as cattle. These areas often have many weedy, invasive plant species. Seeds from these weedy species could easily be carried by the sheep onto your daughter in law’s lawn, and could become established. Once established, they could be very difficult and expensive to get rid of (and since she is renting the property, she could possibly incur some penalties from the property owner).
That being said, I think if the process was thought-out and handled properly, it could be a great plan.