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.
When did Thomas Mitchell begin his explorations during his fourth expeiditon?
On 15 December 1845.

Question: Who else was with the exploration party?
Self-contained rewrite: Who else was Thomas Mitchell with during the exploration party of the fourth expedition?