If you don’t accept the four day week, it's possible you could technically be in breach of the jobseeker benefit conditions in terms of accepting suitable offers of employment and could be disallowed altogether or denied benefit for up to 9 weeks. Family circumstances and rates of pay are factored into how a deciding officer looks at this but there is a risk they could find against you.
I don’t know your full circumstances, childcare costs, travel to work costs, number of children etc., but if at all possible I think you are better off avoiding depending on welfare as you may just get stuck there.
You should do the sums on this, and check out your entitlement to Family Income Supplement and other benefits if you stay in work on a reduced working week.