We start our kids at 3yo.. Mainly cos I'm a bit useless in the sit down and do fun things all day department lol. So in that regard 3yo preschool was fantastic. Socially/emotionally they were probably a bit young, but they loooved all the activities. 2 days a week is really good for helping them settle in. A week between each visit, especially in the beginning is a long time. They get used to the teachers quicker with consecutive days. This proved true with my kids but lots of kids manage 1 day as well.

As for getting a spot at 4yr if not doing 3yr.. I know our preschool gives preference to 4yr olds. So you don't have to be in from 3yrs to get in. If anything there's more worry about not getting a 3yr spot if there's lots of 4yr olds enrolling. They have other preferences as well such as siblings and in-area. It's all a bit complicated heh.