Conversation so far:
What was Mitchell exploring during Thomas Mitchell (explorer)'s fourth expedition?
Mitchell's fourth expedition was into Queensland in 1845-46.
He was convinced that a significant river must flow north-west into the Gulf of Carpentaria.
Was he correct?
Later that year, Kennedy proved beyond doubt that the Victoria did not continue north-west, but turned south-west and joined Cooper Creek.

Question: When did he begin his explorations?
Self-contained rewrite: When did Thomas Mitchell begin his explorations?