I don't think it is feasible at present for you to get a job. 4 dc, childcare costs, baby, mental health issues, no prior job/career experience make this too much of a mountain to climb.
So use this time to budget and do one/several of the ideas given to pull in some extra money. Cleaning, babysitting, dog walking, selling things etc
Sit down with dh and plan a timeline to get back into full time work. 2- 3 years. Nursery costs will be reduced by then and this gives you time to retrain.
You will need to work full time so plan for this. Part time is hard to find and is what people with established careers negotiate for. There are more full time jobs so don't even start considering part time! Focus solely on full time.
Pick a job or career that interests you and has plenty of jobs available. It doesn't need to pay highly but the jobs need to be available.
As well as retraining for a career start to train yourself to be confident and career minded. Posture & confident voice. Challenge yourself to do things you haven't done before. Work on your work wardrobe- charity shops etc
In a couple of years your dc will all be ready for full time before/after care and nursery. They will be absolutely fine.
You have not worked much or had a career before so you don't yet know the payoffs that come from having a job/career. Money as well as confidence and something outside the family to focus on!! You are a capable woman who organises a large family. Start planning for a meaningful job out for the home for the next 20 years that will support your family financially. You do need the money as relying on one person earning (and being in debt) is not fair to any of you. 😊