score:3
Short answer - you don't know until your bus pulls up...
I ended up waiting a fair while at the Cultural Busway station today, and had a chance to ask someone, as well as to confirm by observation what they said.
At quiet times, a bus will always use stop 1, which is furthest from where they enter the platform (by the overpass for leaving the city, by the river towards the city). You'll therefore want to wait at that end
At slightly busy times, a bus will use stop 1 if it's free, otherwise stop 2, so you'll want to wait about half way between the two.
At busy times, all bets are off, and you just have to look and run! Buses will come in in a group, largely due to the traffic lights, and will fill up from stop 1 to stop 4. They won't wait long (at busy times it's pre-paid only which speeds things up), then they pull out and are off. If there were more than 4 buses waiting, bus 5 will wait outside the station until those 4 leave, then drive up to stop 1 (likely with other buses following behind it). Unless something goes wrong, they won't normally try to skip past buses waiting in stops 3 or 4 to get to stop 1, they'll wait for them to go first then drive in. If there are already a few buses stopped when another one arrives, it'll go for the next available stop.
At busy times, you therefore need to join the anxious crowd waiting somewhere around stops 2 and 3. That's far enough forward that you can just about see the numbers of the buses in every stop, but not so far that when the next flight come in you won't have a huge run if you bus goes for stop 1.
There are no announcements, and no information screens to help you. Therefore, if you're slow / have a lot of luggage / etc, your best bet is to catch the bus from either Queens Street or in/around King George Square (as appropriate for the route), where the stops are predictable. At busy times, Cultural Centre busway is a mess and therefore for speedy locals only....