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.
When did he begin his explorations?
On 15 December 1845 Mitchell started from Boree (Buree in Mitchell's journal) with a large party including Edmund Kennedy as second in command.
Who else was with the exploration party?
CANNOTANSWER.
Did anything interesting happen during his exploration?
This rivulet led him to a magnificent pastoral district in the midst of which stood a solitary hill that he named Mount Abundance.
Why did he call it Mount Abundance?
CANNOTANSWER.

Question: How long did the expedition last?
Self-contained rewrite: How long did Thomas Mitchell 's  fourth expedition last?