Our school offered all day-every day for only 1 year and then went back to every other day. That one year was when my daughter was in Kindergarten. I HATED it!!! I know each child will respond differently to it but here is my thoughts: as a former (every other day) Kindergarten teacher and a mother of 4, I believe that kids need time to be kids. At that age, their best learning happens thru play NOT through desk time and not thru worksheets. The absolute best teachers can not possibly fill every day without the use of worksheets, etc. Kids need to be able to move when they want to, not just when its scheduled. They need to play, play, play. (and not with a screen) There is PLENTY of time for them to be students, if given the choice, I would definitely choose to wait with it being all day every day!!
As for your #4 being on the way, I can imagine your feelings. I felt that way when I mistakenly thought I had #5 on the way. The good news is, once you have that baby (and probably long before), you will wonder how you could have lived without #4. BTW-4 is nice! They can buddy up and always have someone.