Mom tells me she doesn’t want to get out of bed and experience another horrible day. Do I let her stay in bed, or insist she get out?

If this is a legitimate question....Talk to your doctor or medical care professional immediately for advice. This is cruel to you and to her.

Hospice for 4 1/2 years is very unusual. While I cannot speak to your Mom's individual case nor the rules of the State she lives in for coverage, this is something that doesn't often happen. In general Hospice comes in for what is "judged" by the MD to be end of life care, not expected to be needed beyond a time of 6 months. It isn't at all unusual for people to be one a year, as it can be re evaluated at the end of 6 months. But 4 1/2 years is unusual.
Your Mom weighs only 70 pounds. The truth is that at that weight she is bound to get a bedsore with inactivity. This is pain to be avoided if possible. So I would encourage Mom to just get up briefly to change position and improve circulation in hopes of avoiding things that can be even WORSE.
Our bodies are made for survival, and the machine fights to live long after the person housed within wishes for peace and rest. I am so sorry. As I said, I would get the best bed possible to prevent bed sores and then let Mom make her own decisions to the extent she is able, but I would explain to her the danger and pain involved in bed sores. In all truth, once they occur they are often the last step before infection, systemic infection and death; Steve Reeves, who was paralyzed following his fall from a horse (Superman?) ultimately died of a bedsore.
I am so sorry for the struggle for your Mom. This is a decision difficult to make. Let her make her own decision after you, and the Hospice nurses, speak with her.

She is 80 years old