Presumably this is private ? Year4/5 is the norm to start around here. Current school arranges to meet parents mid Year 4 to start talking about academic potential possible schools to look at and any unusual deadlines for application, scholarships etc
We went to some open days last year when DD1 was in Year 5. Just to get some idea of whether to consider independent school or the local high school. We have been to more in the last few weeks as she is in Year 6 and we have to make a choice by Christmas. I do not know anyone who has been to open days with children younger than Year 5 to be honest although I guess if you were thinking of moving!
I think children change alot both academically and socially between Y3 and Y5 so their needs would change too.
If you want to go and have a nose feel free they are open to all and you don't have to talk to anyone if you don't want to.You may just get a feel for the place but keep an open mind as they do change year on year.