At this age, I wouldn't worry too much about the friends issue. When my daughter was in preschool at 3/4 most of the kids played independently a lot of the time when in the classroom together, and occasionally played together but it wasn't seen as normal if they played mainly on their own.
Re the differences in systems, I think a lot can depend on the individual school you are in and the teacher you have, no matter which country you are in.
My daughter is 5 and in Kindergarten in the US, but last year we were in the UK for three months to take care of my Dad after an operation. The local primary school let us place her in their reception class for 3 months. She went from going to preschool for a couple of hours, three times a week to another country with a class of new friends and in school from 9am-3.15pm. It was a bit of an adjustment but she really loved it.
It was a gorgeous local school with a hen house in the playground, chicken walking around. Lots of time to play outside, with big bowls of fresh fruit and vegetable to snack on during playtime. Lots of play based learning in the classroom. Once a week they spent the whole morning doing their lessons in a local wood, it was called Forest School. And every Friday afternoon the whole school gathered with all the parents to talk about all the good things which had happened than week and individually praise children from every class. On our last week before returning to the US, the headteacher called my little girl to the front of the assembly and told everyone how great she was, how much they had loved having her there and that she was welcome back at any time and gave her a big hug and a present. It was an amazing experience.
Back in the US this year, we live in a county which only has half day Kindergarten but they are expected to learn everything all the children in the surrounding counties learn in full-day - so there is no recess at all, no break, they have to eat their snack while still working at their desks, no play based learning as there is no time.
So for us the UK school experience at this age was so much better BUT it had a lot to do with that particular school and the direction of the headteacher.