It sounds like members of the public complained to be honest. I am surprised a council was able to do the pilot in the first place given how regulated they are. It's great that they did, just can't imagine it looks good to the folk paying council tax in the area. Councils get a lot of grief for not doing enough as it is, to then know they're getting a 4 day week would definitely rile up some parts of society.
I think we should be piloting this across all sectors, but for a council to be one of the early participants is a risky move.
If the people who pay tax are presented with credible evidence that productivity isn’t harmed, then I’d would hope their reaction is “I want that too”, not “they shouldn’t have it”.