Final answer:
The fourth step in the described reaction mechanism involves the loss of a leaving group, leading to the production of the final substituted product in a nucleophilic acyl substitution reaction.
Step-by-step explanation:
The fourth step of the reaction mechanism described is the loss of the leaving group. In this step, if we consider a nucleophilic acyl substitution reaction, after the nucleophile has attacked the electrophilic center and a tetrahedral intermediate has been formed, the next stage is for the leaving group to be expelled. This results in the reformation of the carbonyl group and the production of a substituted product. The breakdown of the carbon-leaving group bond makes this step crucial in the reaction's progression, as it completes the substitution process, transforming the reactants into their final products.