I took my dd to one of these lastyear.
I thought it was run disgracefully, the children were marched away from the parents and put in a classroom with a bunch of strange kids and teachers and expected to perform.
My dd freaked out at being suddenly taken away from me, to the point whereI was called in to sit with her. She had to do things like draw a picture, sort out different coloured shapes - put yellow in one pile, pink in another etc, then link some toy animals together and the teachers went round chatting to them trying to get them to count a bit and talk about their famiies.
At this school the process was shambolic, the teachers - who were all very young and a - got the children mixed up and were talking to one boy about how he had a older brother at the school when I knew he was an only child. Some children refused to take part, some were having a lovely time. DD behaved pretty disgracefully, refusing to do anything asked, shouting that she hated the school, didn't want to be there and only getting excited when they brought out a plate of biscuits at the end, when she yelled "Mummy, I like this school now. I've changed my mind.".
To my amazement, she got in. I declined the place. Had already decided I wasn't keen on the school, but witnessing this shambles made the decision final.
It sounds like the school you're thinking of will run things a bit more gently and efficiently but my verdict was these things are largely a matter of luck. Some children will cooperate, others won't and prepping imho will only make them anxious. No decent school should mark a 4yo boy down for running around like a loon, it's what they do. Frankly, in the current climate I think you'll have few problems (My dd got in to aseveral allegedly exclusive private schools I registered her for very late. Good luck.