While I have sympathy for your Nanny & feel empathetic that she has a chronic health condition - I also think as your Nanny that her dependability is affecting you + your husband in a way that would mean keeping this Nanny on would just cause you both a ton of stress!
If she has called out 4x in a few months, then that shows that she is just unreliable.
Period.
I sympathize w/anyone who has a chronic health issue, trust me but to find out the morning of that your Nanny will not be coming in will effect you and your spouse’s job performance which is unfair.
Did your Nanny let you know about her chronic condition prior to hire?
Or did she began having health issues shortly after beginning her position w/your family??
Perhaps your Nanny would be best suited for a part-time position or even one where the family can be more flexible.
It just sounds like an unsuitable fit and it would be best if you began looking for a brand new replacement.
Good luck!