The client represents the 'portal of entry' in the chain of infection, which is the point where pathogens can enter the host. Therefore, the correct answer is number 4) Portal of Entry.
The client who acquired an infection from a visitor can be considered as a portal of entry, which is an anatomical site where pathogens pass into host tissue. In this scenario, the client's body served as the entry point for the pathogens brought by the visitor. The chain of infection includes various links such as the reservoir, portal of exit, mode of transmission, and portal of entry, with the client being an example of the latter.
In conclusion, the correct answer to the question is: 4) Portal of Entry. Given that the infection was acquired from another person, the client became the next point in the chain of infection through which the pathogen entered.