Conversation so far:
What was Thomas Mitchell exploring during his fourth expedition?
Thomas Mitchell was convinced that a significant river must flow north-west into the Gulf of Carpentaria, this being the main thrust of the endeavour.
Was Thomas Mitchell correct in believing a significant river must flow north-west into the Gulf of Carpentaria?
Thomas Mitchell's second-in-command 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 during his fourth expeiditon?