At the time this story was written, how much was 4 British pounds a week worth in American money?

There are several ways to try to evaluate how much money the phony Red-Headed League paid Jabez Wilson for copying pages of the Encyclopaedia Britannica. 

The exchange rate for American dollars to British pounds in 1891 was $4.86 to 1. Therefore, Jabez earned $19.44 per week for his skills as a scribe.That doesn't seem like much, but we have to consider the effects of inflation. A dollar in 1891 would buy much more...

There are several ways to try to evaluate how much money the phony Red-Headed League paid Jabez Wilson for copying pages of the Encyclopaedia Britannica. 

The exchange rate for American dollars to British pounds in 1891 was $4.86 to 1. Therefore, Jabez earned $19.44 per week for his skills as a scribe. That doesn't seem like much, but we have to consider the effects of inflation. A dollar in 1891 would buy much more than a dollar in 2016. Plugging $19 into an inflation calculator website (below) shows that the equivalent amount in 2016 is $487, so with rounding we can say Jabez was earning the equivalent of $500 per week in 2016 American dollars. He was working about 20 hours per week for the League, so that was a rate of $25 per hour. Thinking of that another way, he was earning about 3.5 times the current U.S. minimum wage, the wage one might expect to earn for unskilled labor such as Jabez performed. Jabez was sorry to see his gravy train end; if he had been able to keep up his duties for an entire year, he would have pocketed $26,000, a significant boost to his income.

Although significant, even that amount doesn't really convey the plush position Jabez thought he had landed by virtue of his red hair. One can also consider the "economic status value" of that amount in 1891. This measurement takes into account the comparative earnings of others in the society. According to Measuringwealth.com, the economic status value of 4 pounds in 1891 is equivalent to 2917 pounds today, which equates to $4053. That means that his economic status compared to those around him would be similar to someone in our society earning $210,000 per year--from part-time work. Now we can see why Mr. Jabez Wilson considered the loss of this employment a "grave enough" problem.