well, if your bunny is already four or five and has been a single bunny her whole life, then i would not worry about her being lonely. she is accustomed to her own time, space, and territory now.
you should not feel bad about her being without a friend. rabbits can live very long and happy lives without a companion.
however- if you do decide to get a new bunny friend for her, i would strongly advise you to get a new bun that is a male, as it is much easier to bond a neutered male and spayed female. this has been my experience.
and also- always be prepared that you may have to keep them seperated forever in case they do not get along. this is always a possibility. some rabbits just refuse to bond.
Rabbits are extrremely territorial - if you and your home is all your Bunny has ever known- she may very well take a lot of time to get used to the idea of another rabbit in her space. Also- some rabbits are fine with other rabbits when meeting them in a neutral place like a shleter or another house, but once you bring them back home, your original bunny will act differently. Keep a very close eye on them, as major injuries can occur rapidly even if it's just from sniffing between cage bars. lips and noses can be torn badly.
however- some rabbits love eachother immediatley- which is why i suggest getting a male rabbit for your female. also keep in mind the age differece. you may want to try and get another rabbit close to your bunny's age if you are concerned with companionship, so one is not left behind at a young age.
well- that is all i can think of for now- but whatever you decide- i wish you the best of luck.