It depends on the job.
I'm always wary of the idea of working 4 days a week, as far too often that means you are expected to do 5 days work for 4 days money.
When working 3 days a week, it is more likely to be seen as PT with the other two days work covered by someone else, or not allocated to you.
This will, of course, depend on the nature of the role.
If you can be sure you will only get 4 days work, then I'd do the calculations as suggested above.
The way personal allowance works, your net pay for working 3 days a week is quite a bit more than 3/5 of FT pay. You'd have to make sure your calculations are right about your net pay and then take off the additional Nursery costs (which I've lost track of now, with the Gvmnt paying for 30 hours and so forth).
Then, remember this isn't just about this year's income, but also about pension contributions in the long run.
Plus (none of my business, but something to think about) what you would get if you were planning to take another maternity leave in the future.
As to 'how much time' - lots of things get factored in to this, from how long your commute is each day, to if you are able to wfh sometimes, to whether yours is a job that you finish when you leave the office or if it is one you do additional work at home, or that you are worrying about constantly when not there, or if the new role involves travel or even nights away, etc. These things very hugely in terms of work / life balance.