How has Bilbo Baggins changed since the beginning of the book to the fourth chapter?
In Chapter One of The Hobbit, Bilbo Baggins is described as a peace-loving person who enjoys the comforts of life, living in his beautiful hobbit home in the side of a hill. He loves food, his pipe, and an occasional stroll around.
All this is disrupted by the visit from Gandalf and the party the dwarves had in Bilbo's house. He is very disturbed by the chaos and upset with the unexpected guests. He is glad to get rid of them. Yet, somehow, he finds himself following them the next morning and embarking on an uncharacteristic adventure.
While the dwarves don't think much of Bilbo, by the fourth chapter, he has already saved them from evil trolls, surprising them and himself. The completely unadventurous hobbit has become an indispensable member of the group. In chapter four, adventures continue and Bilbo ends up separated from the party; he will soon acquire the ring that will become part of his nephew Frodo's long adventure.
In short, Bilbo Baggins at the beginning of the story is completely unadventurous. He becomes a reluctant adventurer at the beginning of the journey but also the person who saves the rest of the party more than once.