When I worked a 4 day week I really did work a 4 day week but that in the days before smartphones and standard working from home and so when I wasn't in the office, I wasn't available.
Later I worked 4 days spread over 5, and I did check emails and answer them out of hours. But, you have the power not to. You can go out, do what you do what to do, and look at them on your own terms. If you choose to answer on a day off to help colleagues out, that's up to you. But you are not being paid for those hours, they are (presumably) being paid full time, so are being paid to cover when you are not there.
Part-time working isn't the same as flexible working, a part-time job can be very inflexible, but for me, the pro's of flexible working completely outweigh any cons.
I work 30 hours per week Monday to Friday, 9am until 3pm. Outside of these hours I have childcare responsibilities, therefore I am unable to check and respond to emails and voicemails
Almost, don't mention childcare. I would suggest
I work Monday to Friday, 9am until 3pm. I will respond to emails and voicemails during those hours