What type of visa should I apply for to stay in Thailand for 3-4 months?

For a stay of 3-4 months in Thailand, you can opt for a 60-day visa exemption upon arrival, which can be extended for an additional 30 days at an immigration office for a fee of ฿1,900. Alternatively, you could consider doing a border run to renew your visa exemption, allowing for a similar process again. It's suggested to check with the Thai Embassy in Australia for detailed inquiries.