Praise be to Allah.
If she did not fast because she was not able to, then she has to make up the fasts she has missed in these four years, when she becomes able to. Allah says (interpretation of the meaning):
“and whoever is ill or on a journey, the same number [of days which one did not observe Sawm (fasts) must be made up] from other days. Allah intends for you ease, and He does not want to make things difficult for you. (He wants that you) must complete the same number (of days), and that you must magnify Allah [i.e. to say Takbeer (Allahu Akbar: Allah is the Most Great)] for having guided you so that you may be grateful to Him” [Al-Baqarah 2:185]
If there is no hope – according to her doctors – that she may be cured of her sickness or that her inability to fast will ever be lifted from her, then she can feed one poor person for every day that she misses, giving half a saa’ of wheat, dates, rice or whatever her own family regularly eat in their home. This is like the case of the elderly and disabled who would be too exhausted by fasting or who find it too difficult to fast. In this case she does not have to make up the fasts she misses.