Need Veneers for All Four Front Teeth or Just the Two with Chips?

There are many factors to deciding how many teeth to treat.  Porcelain mimics enamel, but not PERFECTLY.  In some lighting, it may be noticeable that 2 teeth do not match.  By doing 4, the difference is less apparent.  If you want whiter shading, then more is needed.  If you are happy with the color, perhaps only 2...
Also, veneering allows reshaping and contouring, so if the adjacent teeth are short or long, they can be changed.
Ultimately, the choice is yours.  If you feel you will be satisfied with just 2, then that should be fine.  Clear goals and expectations should be defined well in advance of treatment.

If you are happy with your other teeth then don't touch them. The dentist may have noticed things about the other two teeth that you haven't such as asymmetry or different colors. 

What the dentist thinks looks good and what the patient like are sometimes different. 

You can always get the other two done later if you think it is necessary.

If you have chosen a skilled cosmetic dentist, they will be able to evaluate all the aspects of your smile and help you decide how many veneers is right for you. It sounds like they haven't spent the time they need to let you know what is best for you. 2 veneers can restore your teeth to what they were like before you broke them. 4 veneers may enhance your smile if it is deficient. 8-10 veneers can give the doctor greater control and possibly a "Hollywood" look for you.
You need to consider what you need and then think about what you want. You need to restore broken down teeth to maintain function and health. You may want to enhance your smile while you are spending the money anyway because it is easier to design all the veneers at the same time.
The reason to get 4 veneers would be to enhance the shape of 2 teeth adjacent to the front teeth. I would recommend that you get a mock-up done so you can see what it will look like to have 4 veneers. When I do this, I use composite and sculpt it around the teeth to show the patient what it will look like. If the dentist won't do this, I would recommend getting a second opinion. It is always nice to see it done before something permanent is done.