I am not currently looking for a new job but I am thinking about the future. In my current job, I negotiated a 4-day work week (for a proportioned cut in salary) a year or so after starting there.

My reasons for doing so are mostly that this schedule offers me a better work-life balance, reduces stress and lets me enjoy life more. If I was required to expand on this, I would even argue it has positively affected my productivity, but I am not here to argue the pros and cons of such a schedule.

I would like to continue with such a schedule in the future. When looking for a new job, when is it a suitable time to mention this? On my resume? The first interview or phone call? After I have received an offer?

Many employers can see this type of thing as a negative, which is why I would be reluctant to mention this before receiving an offer, but at the same time I respect the employer's time and do not want them to waste it.

Additionally, when seeking a new job I sometimes deal with recruiters; should it be something I mention to them or no?

If it matters, I am based in the UK, in the software industry.

I would like to continue with such a schedule in the future. When
looking for a new job, when is it a suitable time to mention this? On
my resume? The first interview or phone call? After I have received an
offer?

That depends on how important a four day work week is for you, and how flexible you are. Let's assume it's a must have and you would not accept a job without that perk. In this case you'll have far fewer opportunities, but those will be companies that could actually meet your needs.

First, you should be looking for job listings that talk about "flexible", "work-life balance", or of course "four day work week". These kinds of companies are more likely to meet your needs.

Second, you should be ready to ask the question during your first interview - either on the phone or in person. Since this is a go/no go question for you, you'll want to avoid wasting your time and their time if they cannot accommodate your needs. Ask soon into the process.

Third, you'll probably want to be ready to explain why this is important to you, and also why it would be good for the company. Remember that many companies care far less about your work-life balance, stress, and personal enjoyment and more about the work they need to get done and your productivity. So be ready to convince them why this would be good for them.

If this perk isn't critical for you, then you could take other approaches. The one most likely to be successful appears to be one you have already used: find a full time job, work there until you are trusted and respected, then ask to go to four days per week.

Additionally, when seeking a new job I sometimes deal with recruiters;
should it be something I mention to them or no?

Again, assuming this is a critical perk for you, then that is the first thing you should mention to any recruiters. They will know (or can find out) if the hiring company would be willing to be flexible.

Yes, you can. But you might find that your prospects are limited in preference to people who are willing to work a five day week. Your prospective employers of course don't know that you work 20% more efficiently than anyone else in your team.

You'll probably need to agree to the five day week and then negotiate from there when you've proved yourself.

Another option is to negotiate regular work-from-home days. I work two days a week at home and it's an arrangment that's helped me no end in my work-life balance. As long as I get my work done, people don't mind if I'm working at 7am or 7pm.

While it might be possible to find an employer who's happy for you to work the four-day week immediately, I'm not sure it's going to be likely.

Here in the UK I've worked with contractors who do a 4 day week using the extra day to travel back home. In one case they have actually done 4 days * 10 hours (assuming a 40 hour working week). I have also had a friend who negotiated a day off for similar work life balance issues. Certainly in the London City job market it's not too unusual for folks to come in on Monday late and leave early on Friday to do a long distance commute home after spending the week working away.

So it's certainly possible but you will potentially be limiting your options. As someone who has interviewed candidates I have been asked this question a couple of times I've usually replied that it is possible (as is remote work) but I wouldn't guarantee anything and certainly during any initial probation period we'd probably expect folks to be in the office full time. So far I haven't had any candidates we want to make an offer to ask the question so I'm not sure what would happen in reality.

Personally I suspect we'd be quite happy to make an offer but you would have to be a really strong candidate OR willing to do 5 days initially.

Oh and don't bother mentioning it to recruiters, if nothing else the interview practice is good.

Rather than mention it, specifically target companies that offer a four day week, or better yet, tout it as a benefit.

It would be far easier to join a company where a four day work week is already part of the corporate culture than it would be to shoe-horn yourself into a company where that culture does not exist.

Should you just "cold call" on the 4 day thing, then you need to mention it early, and have some very good reasons why it would be worth it to them to have you on for four days.

Be able to answer the question:

"Why would we want to hire someone who's only going to be available for half the week?" or something similar.

If it is important to you, then mention it early in the interview process.

Not necessarily on the phone screen, but definitely at the first in-person or video interview.

How you phrase/present it is important.

Don't say, "I want a four day work week."

Instead try something like: "After working at my current company for a while, I changed to a 4-day work week. They have been especially pleased because my productivity actually increased, and they are only paying me based on a four day week."

If that is a definite deal breaker, they will tell you right then.

Remember that you need them to know this because your current salary reflects pay for a four day week. Bring it up early enough so you aren't presenting in a 'take it or leave it' way, which gives them time to process the information.

If they reply, "Is this a deal breaker for you?" you can respond that it is a "strong preference" especially since this is your current situation.

You don't come across as inflexible, but they can figure out that they need to consider if this is an option if they want to keep talking to you.

I would do the same with a recruiter - you don't want to go to a recruiter's interview and have them hear this information back the company. Could make them look bad - and as they tend to make themselves look bad often enough, that can really annoy them ;-)