Once you drop the hours, is it easy/hard/very hard/impossible to go back up should the need arise in the future?
Would you be under pressure to come in on day 5 (to work, attend meetings, answer emails at home, do training etc), or are the systems robust enough for that? (Some roles in NHS I presume are very much "come in and cover your shift, handover, go home", while others are "this is your pile of work, get it all done").
Have you looked at the household budget, and would it be manageable if you drop the day?
Then, on the far more positive side:
How much will you gain by doing it?
How much time will you free up for getting on top of the house and DH, doing bulk cooking for working days, getting a chance to shop properly?
How much time will you free up to devote to DH and improving his moods, lifestyle, outlook?
How much time will it give you to take the foot off the pedal slightly, and take even a few minutes for yourself every day?
From my own perspective, I took parental leave to do a 4 day week for 5 months a couple of years ago. DD was struggling (ASD/ADHD), I was way over-stressed at work, DH was travelling all the time meaning a lot was falling to me. I am not the main breadwinner, but I have always been under pressure as having the permenant and pensionable job (DH pays into a private pension, which sucks earnings-wise, and his job is always slightly vulnerable due to the way the economy is). So stepping off entirely was not an option.
I know that it made a huge difference to me coming back to being myself - I am still massively stressed but it gave me a chance to catch up on life and give DD more time too which helped her settle back onto a much more even keel. I am now back 5 days a week, but it was well worth it for that break.
If you would be able to step back into 5 days should you need to, and can manage financially on the lower income - I would say yes. It may help you to be the strong person you need to be.