What is value in dollars of 4000 pounds sterling in 1850?

One way to work this out would be to find out the price in 1950 of gold/silver, work out how much Gold £40,000 would have bought back then and then apply today's gold prices to arrive at the answer. In 1950 the average price of a troy ounce was $34.72. The GBP/USD exchange rate was roughly .42p to $1 so £40,000 was equivalent to $95,238. This would have bought ~2743 troy ounces. At today's gold price ($1245) this would be equivalent to $3,415,072 or £2,276,714. Obviously these are not exact but should be more or less correct.