Final answer:
The 1,4-addition of HBr to 1,3-butadiene results in the formation of 4-bromo-1-butene, with the hydrogen and bromine atoms adding to the first and fourth carbon atoms respectively.
Step-by-step explanation:
The compound formed from the 1,4-addition of one equivalent of HBr to 1,3-butadiene is 4-bromo-1-butene. During the reaction, the HBr adds across the 1 and 4 positions of the 1,3-butadiene molecule, with the hydrogen atom attaching to the first carbon of the diene and the bromine attaching to the fourth carbon. The reaction involves the formation of a more stable allylic bromide intermediate which is more substituted, leading to the formation of 4-bromo-1-butene as the final product.