With this in mind, realize then that it is extremely rare for someone who gets the vaccine virus to spread it to another individual who does not have any protection against chickenpox. In fact, there are only three documented cases of chickenpox out of more than 15 million doses of vaccine given to date that were transmitted to healthy people from recently vaccinated people. These three cases ended up being mild disease and there were no complications. More importantly, this type of spread has only been documented when the person receiving the vaccine develops a chickenpox-like rash right after their shot.
Since the chickenpox vaccine was licensed in the U.S. in 1995, recommendations to promote wider use of it have expanded. Some states require children entering school who have not had the disease before to receive the immunization. Chickenpox vaccine is not recommended for children who cannot fight infection well, have cancer, are taking large doses of steroid medicines, or for women who are pregnant. So the bottom line is I would suggest your 4-year-old receive the vaccine, assuming everyone is healthy and well in your family. It is very unlikely she will spread it to her infant sibling.