How many school hours for a four-year old?

It recently came to my attention that some Montessori schools including in Australia have four-year old children at school for five full days a week. 

Caspar is four but only attends school for three hours a day, five days a week. This is the norm for his school when students don't attend full days of school until they turn five (or close). Until now. His school has decided to trial early extended day whereby children are invited (it's not compulsory) to attend two full days a week, in addition to the other three half days, two terms before they turn five. 

We had to make a decision. Did we want to participate in the trial and increase Caspar's school hours?

As parents it's difficult not to feel torn. One side of me doesn't want him to grow up too fast and wants him to stay at home for as long as possible. The other side of me wants to increase his independence and his participation in the wider community. In the end we decided to go along with the trial and from next term (only weeks away!) increase his school hours. 

Caspar is actually excited and can't wait!

I am interested to know, as I've only heard from those supportive of the increased hours, what is typical in a Montessori school for four-year old children? Are most four-year old children attending five full days of school? I'd love to hear your perspective or that of your school.