Given what a typical 4 year old knows about death, it most likely means nothing, as painful as it may have sounded to you. You know about suffering and death; she does not, especially if she has never experienced someone (or something) close to her dying.
According to Child Bereavement UK, children 2 to 5:
Often struggle with abstract concepts like ‘forever’ and find it difficult to grasp that death is permanent. Their limited understanding may lead to an apparent lack of reaction when told about a death.
Agreeing with this, the American Academy of Pediatrics states that depending on their age, development, personalities and other factors, children may express (among other things):
Loss/emptiness/sadness, Relief, Guilt and self-blame, Fear, Acting out behavior, Explosive emotions, Regression or Lack of feelings [emphasis mine]
The article goes on to state that at ages 2 - 6, dead only means "not alive”, that it's not concrete yet, and death is seen as temporary.
Ages 4-6 years: Many of the characteristics of the 2-3 year old development remain, particularly a belief that death is temporary, and the loved one will be returning (or even that they should go visit them). What is additionally challenging at this age is the tendency to think that they did something to cause the death, so called “magical thinking.”
The linked article has materials to use in explaining death, et., if you need them, however, it will probably not mean much to your daughter until she witnesses your sadness and the dog's decline. This is a great opportunity, however, to teach your daughter about the natural aspects of death.
I'm sorry for your loss. I have senior dogs and the thought of their demise is very sad.
Coping With Death
How do I explain to a 4 year old that our family pet is sick and needs to be put down?