1. weather - can check
www.bom.gov.au. It has everything you need.
2. you should wear summer clothes during the day and definitely something warm at night - our night time is cold and windy. you will be used to it!!! i will bring a fleece jacket.
3. Any hostel is fine - they should be in the CBD areas ... it will be good for you to take public transportation.
btw, 1 month to go to 4 cities in Australia - means you wont see much and only the cities and a little bit of surrounding areas. Australia is a big country. Travelling to places takes a long time.
If i had one month, i would concentrate on one state only and take it slowly. there are many places that takes a buses and ferries to get there. cannot fly direct ... this is where you should go ...
If i go to Cairns, i would go to WhitSunday islands or Hamilton Island ... i will travel along the coast and definitely will go to Fraser Island and fly back from Brisbane. This will take you about 1 month without haste. If there is still some time, you can travel further down south to Sydney ...