The Ingles from Ferrol is 112km long. Meaning, for a 4 day walk, you need to be doing almost 30km every day. The 'guidebook' stages of the Ingles are for a 5 day walk, meaning that you are planning to walk quicker and longer stages than many guidebooks, and many people do the Ingles over 6 days. The question is, will your wife and mother in law be able to do this?
If a 4 day Ingles were to be planned, I would go for the following:
Day 1 - Ferrol to Pontedeume, but walking across the N-651 bridge shortcut from Ferrol to Fene - about 7 hours walk - 26km
Day 2 - Pontedeume to Betanzos - about 25km - about 7 hours walk.
Day 3 - Betanzos to Buscas (this is about 1 hour past the Hospital de Bruma albergue) staying at Casa Rural Donnamaria - about 32km walk - 10 hours
Day 4 - Buscas to Santiago - about 30km walk - 10 hours
Good, non-hostel accommodation can be found for all 4 nights.
First two days are tough if you are not fit and have not had training. Last two days are tougher still but doable. The Ingles is also not flat.
Whilst the above schedule is feasible, it does not give much scope for enjoyment. If the Compostela is not an option, I would consider what Rick has said, of walking in Ireland on the Celtic Camino and then from La Coruna to Santiago on that Ingles branch, walking from Santiago to Finistere, or just walking 4 days on the Frances as
@marylynn suggests, not necessarily ending in Santiago.