My son just turned 5 and we've had a few not-so-close family members die (nothing as closely related as a great-grandmother) and some pets.
My son first started mentioning "death" around age 3 1/2 or 4 probably and I was a little surprised at how much he understood about death in general (that it means that the person or animal has left and won't be returning). Where he learned this stuff, I have no clue or if he just sort of figured it out organically.
My general rule when discussing stuff like this with my son is to keep it simple, know what points I feel need to be covered, and more or less allow him to lead the conversation once I've introduced it. Abstract doesn't work for 4 1/2 year olds--they are very concrete individuals at that age and being vague or abstract in how you approach it is likely to lead to more confusion and, ultimately, more questions. Don't get me wrong--questions are great, they just might not be the type of questions that are especially helpful to the situation. Does she know anything about death at all? Having to explain death in general is a different situation than having to simply insert a loved one's death into her all ready constructed framework. If you don't all ready know, then ascertaining this might take some gentle probing or maybe some creative role-play with some Barbie dolls (oops, Barbie fell and is "dead" and see how she responds to that. I know, it's macabre, but it might give you more insight than simply asking her what she knows).
If you have any particular religious beliefs, then now would probably be a good time to insert them. If not, then now would be a good time for you to decide how you might respond to questions like, "Where did grandma go?" "Will we see her again?" "Why did she have to die?" etc. She might not ask any of these questions. She might ask these and more, but at least if she does ask you will have a pretty good answer even if it's "I don't know".
If you plan on taking her to the funeral/wake/burial service/etc., you might want to prepare her for that. It could be disconcerting for her to see her great-grandmother lying in a casket, or for her to see other family members obviously upset and/or crying.
My best friend's father passed away a year ago and it wasn't until a couple of months after the funeral that her daughter (who was 5 at the time) really started asking questions about death and dying. So even if she doesn't say much now or ask many questions now, she might in a few weeks. As an aside, my best friend's father was cremated and she still hasn't shared that bit of information with her daughter.