Hi Mark, lots of questions here!
Firstly let me qualify this by saying that I have a bit of experience with this but you should get everything verified by a tax accountant before doing anything.
The tax office would probably consider you to be in the business of developing as this isn't exactly a small venture.
Whether you're deemed to be in the business of developing or not really changes your tax situation.
The rest of my feedback below assumes that you are in the business of developing.
Your profit in the first year if you keep unit # 5 (by the way, you wouldn't own the body corp, you'd only own 1/5th of it if you only own 1 unit) would be something like this:
Sale of 4 units $1,000,000
Plus market value of unit 5 $250,000
Less cost of land & building costs $1,000,000
Profit of $250,000
It doesn't matter that you haven't actually sold unit # 5, you'd still have to include it in the figures because it's treated as stock on hand.
You also need to consider GST because being new units, your sales price would have to include GST.
This is a whole new discussion which you can't ignore because it could reduce your profit considerably.
There is no CGT if you're in the business of developing, so therefore no CGT concessions if you hold greater than 1 year.
You just pay tax on the end profit. The tax you pay depends on who owns the property.
If the property is in a company, you'll pay 30% on the profits.
If the property is in a family trust, you have the flexibility to distribute the profits to family.
So the idea is to distribute the profit to the lower income earners to keep the tax % down.
Sorry, can't help you re: the tax on the subdivision.
Sounds like you need to make an appointment with your tax person.
Keep at it. It sounds hard now, but once you've got your head around it, it's not that bad.
Good luck.