It is not uncommon for me to hear from a client, "I haven't taken him out yet." This refers to a puppy that hasn't been outside for the first 3-4 months of its life. It would be like having a newborn and taking her for her first stroll at six months of age.
Dogs need to be exposed to the outdoors, and all of its sounds and sights, at a very early age. In so doing, the risks of poor behavior (behavioral problems being the single greatest reason for which a pet is euthanized), is greatly reduced. Try to do this at 4, 5, 6 months of age, or older - forget about it: you'd be setting the dog up for a life of phobias and anxiety (devastating for both you and your pet).
A typical vaccine schedule for a dog is 8, 12, and 16 weeks. How do you ensure good behavior and protect the dog from infectious disease? The short answer: you don't. The risk of the dog acquiring fatal infectious disease is mitigated by the huge risks of your dog developing behavioral issues if she's not taken out early in life.
Have common sense: expose the puppy safely, to dogs whose owners you know, or are sure are vaccinated and not exhibiting symptoms (such as coughing, diarrhea,... you get the picture).
There is no excuse for the four-month-old puppy who cowers behind his owners upon
meeting another dog for the first time. At that age, it would be like running into this: