I think that its great you want to help a shelter rabbit.
However, keep in mind that bonding a rabbit to an existing trio is a very difficult and lengthy process and there is always the possibility they won't bond. Are you prepared to keep the fourth bunny as a separate bun?
When bonding multiple rabbits, according to my research, its usually advisable to do one on one bonding. Like Stichnwinks suggested start with the most dominant bunny.
You'll have to give the new rabbit plenty of time to adjust in her new home before starting bonding as meeting so many rabbits will be very stressful. Shell also need to time to settle her hormones after spaying, if she isn't already.