Is the incubation period (infection to symptoms) for common viral/bacterial infections the same in a 4 week old as an adult?

Yes. The incubation period for viruses and bacteria are related to the organism in question, not the age of the host.
Yes. It depends on the bacteria or virus and not the age of the person.
Incubation period. Incubation periods do not vary by age, how severe the presenting symptoms are does vary by the age of the patient. Remember most incubation periods reflect a SPAN of time between exposure and presentation and are not usually exact.