What to do with no work history for 4 years?

If I hired someone, and found out that they lied during their interview about anything at all, they'd be fired, period. I could not trust someone who lies during an interview to make ethical decisions in the future. Honesty doesn't have to mean full disclosure. You can be honest while keeping some facts to yourself.

Here's a good example:

I was taking some time off for voluntary reason when I became ill. I was lucky enough to be able to afford treatment and to delay my re-entry to the workforce until I became confident that my health wouldn't be a hindrance.

Just keep it simple and be honest.

I realize that there's a stigma attached to certain types of illnesses or disabilities, and it may seem tempting to make an excuse or cover rather than being honest. However, in addition to the likely inevitability of the deception being discovered, there's a worse possibility:

One young man I know (Calling him Bob, not his name) interviewed with a friend and colleague of mine (calling her Alice), but didn't get the job. I checked in with Alice to find out what happened, thinking that at least I could get Bob some useful feedback. It turns out that Bob had rated well on skills related to the job. However, he'd been so cagey about a six-month work history gap that the interviewing committee thought it likely that he'd worked some job not on his resume and was fired for cause, perhaps for stealing or sexual harassment or "something else big".

Bob was so afraid of the company finding out that he'd been in psych care after a major trauma, that he led them to believe he was a criminal.

Honesty really is the only good policy.

OP has no responsibility to share about their illness. In this case they are perfectly within their right to tell some half truth (wanted to spend time with my family, study x subject, burnout etc) and your company might be in some legal hot water if you fired OP in response to finding out they were ill during that time..

Frankly I think it's unethical, and shows a lack of empathy, to punish someone for hiding an illness from an employer.

I think this answer draws the right balance between truth and vulnerability.

A corporation is not your friend. The interviewer is probably feeding you less benign little lies about the company simultaneously.

That's an odd lesson to take away from your anecdote about Bob. What Bob should have done was just told a white lie that wouldn't have raised any red flags or follow up questions, and left it at that.

There will always be a stigma around mental illness, it's human nature, not some social construct, and I would 100% lie to you about something like that and not feel the least bit bad about it.

I would stick with the truth. The truth doesn’t mean the whole truth, but some subset. At minimum you will feel better if you don’t lie, but worst case you get the job and your manager later learns there was no relative. You’ve abused trust and can harm the relationship.

You are allowed to be ill. You don’t have to elaborate on the condition. Just emphasize it was serious enough you were unable to work. Assure them you’re recovered and hence now looking.

But I would avoid the impulse to lie. It doesn’t matter if everyone else is lying around you. Or if corporations lie. Or whatever. There’s literally nothing to be ashamed of or to hide, so why descend to a level beneath yourself to hide something that doesn’t need hiding?

Finally there seems to be a view here that all managers are out to get you or extort you and will lie and cheat to get it done. I’m a hiring manager of hundreds and I genuinely do care about my people as people. When someone is sick I appreciate what that means. You shouldn’t want to work for someone who doesn’t. If the only way to get their job is to lie about something you shouldn’t have to hide, why on earth would you want their job? There are better people out there hiring. Even if it takes a little longer find the RIGHT job not the expedient job.