Personally, I would find working 4 hours a day for four days a week a right pain, all the hassle of getting up and out, to only do 4 hours, then home, by the time you get there, it's after lunch, bit tired, then it's evening meal etc. I suspect your life would be less monotonous if you did two full 8 hour days, and 5 days off, which is a lot of time off.
I guess that also every day and every weekend is very similar.
I think the person who suggested dividing up the running about for your dd was onto something, if you did two eves, your husband did one and the Sat, that would work well and free you up a bit.
Add in more things for you- choir, groups, what do you like to watch on TV, meals you like, things you choose for yourself. Any hobbies you've wanted to get into- language learning, plants/gardening, writing, start to do even one half hour every afternoon, you'd start flying along once you have set your goals and some concrete targets.
I think the problem with these part-time jobs is if you make them feel full-time and don't plan out the rest of the time, they end up being almost as much trouble as full-time work. You need to stop that and start seeing the benefits of having a three day weekend (so one whole day for decorating/your own stuff, one day your husband runs your son/dd to dance and one day with the IL's) and also use those afternoons a bit rather than just scrolling on here or on the internet (I block this site most of the time except in my breaks, and still get to post quite a lot!)