Letter IV mostly narrates the early part of the friendship between Captain Walton and Victor Frankenstein. As Victor recovers from his brush with death on the ice, he and Walton have a great deal of time to talk, and they find that they get along very well and begin to really care for one another as brothers.
Walton takes this opportunity to open up to Victor about his goals for this dangerous expedition to the Arctic. He longs to find the Northwest Passage, a waterway that connects the northern Atlantic Ocean to the Pacific Ocean and to understand the mystery of how the compass works. Discovery, in general, is Walton's primary goal, and he yearns to make a significant contribution to humanity's knowledge and thereby confer an "inestimable benefit" on "all mankind to the last generation" (Letter 1).
As he communicates these desires to Victor, Victor expresses a great deal of understanding and sympathy for Walton's ambitions. Then, Walton says,
I was easily led by the sympathy which he evinced to use the language of my heart, to give utterance to the burning ardour of my soul and to say, with all the fervour that warmed me, how gladly I would sacrifice my fortune, my existence, my every hope, to the furtherance of my enterprise. One man's life or death were but a small price to pay for the acquirement of the knowledge which I sought, for the dominion I should acquire and transmit over the elemental foes of our race.
When Walton suggests that he would be willing to sacrifice everything else he holds dear, including his own life, for the sake of knowledge and discovery, Victor begins to cry. When Victor finally speaks, he says,
"Unhappy man! Do you share my madness? Have you drunk also of the intoxicating draught? Hear me; let me reveal my tale, and you will dash the cup from your lips!"
When Victor asks if Walton, too, has drunk the "intoxicating draught" that Victor himself as drunk, we can understand that Victor is using a metaphor to describe the desire for personal glory, the dream of which is so "intoxicating" (like alcohol) that it makes the dreamer forget everything else that makes life worth living. For the sake of personal glory, Victor has, essentially, lost everyone he loved and he will soon lose his own life. To hear Walton say that he is willing to lose everything for the pursuit of his one ambition makes Victor realize the startling similarity between himself and Walton, and as his friend, Victor wants to spare Walton the pain that he has gone through.