OP I'm in NSW and have taught K-2 for the last 13 years. I've seen it all from 4.5 year olds to 6 year olds starting school. 4.5 year olds just don't tend to cope well with school in my experience. We have huge expectations on kindy children right from the start. Out of interest do you know if your school teaches L3 for literacy? We really push them hard right from the start (not by choice, but to get their reading levels to a certain point by the end of the year). My DS2 will be turning 5 in February next year and will most likely be held back. I am concerned about him managing the expectations that we have at school when he's just turned 5. Boys in particular benefit from the extra year.
I understand you seem fairly certain about sending him but my experiences with 4.5 year olds tells me that they are definitely not ready to cope with the expectations of formal schooling.