Essentially, 4 days a week in school means that you don't have to work at the weekend - and as a parent you may find that compromise worthwhile.
Full-time, IME every primary teacher who is a parent (ie cannot work indefinitely after school in the week), and many who are not parents, spend at east part of every weekend working - whether it be marking the writing from Friday in depth, tweaking the planning for the following week, doing anything to do with their subject responsibility etc.
5 days a week moves that weekend work firmly into the week and gives you a fully free weekend (except that colleagues who are working may well e-mail you, if you do any form of joint planning etc) and also gives you 1 early finish - so you will be able to fit your 'displaced weekend work' into a 9-3 day.
However, you will also have to factor in handover time - about an hour a week minimum between a 3 days / 2 days jobshare, IME, possibly a little less between a 4/1 - becauise it is common for heads to want an absolutely seamless experience for the children. think also about practicalities of English and Maths books, which if your scjhool is anything like mine, always have to be marked for the next day's lessons but will also need to be available to your jobshare.
1 day jobshare rroles are also notoriously hard to fill with high quality teachers, unless there is an existing e.g. PPA cover teacher who can do a full day in your class and other time in others.
One option that can be surprisingly successful is 5 mornings / 3 afternoons - so 0.85ish, in a normal school timetable. It avoids handover issues with English & Maths, and means that if you are disciplined you get 2 3 pm finishes and still avoid weekend work - it works best if you live close or can stay in school to work, though.