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

If you are hiring an experienced cosmetic dentist for the job, here are some things he/she may be considering. First if there is currently any asymmetry in your front 4 teeth, and you go and make the front 2 teeth symmetrical (which you will with your veneers) - you will amplify the asymmetry remaining in the 2 side teeth. Also, if your front 2 teeth are currently larger or have any spaces, these teeth may end up looking too large without doing the side 2 teeth. If your side teeth are symmetrical, proportional, and the color you want them to be...and the front two teeth are proportional in your smile, you may be able to get away with just doing 2. You may end up with several more visits as you try to get a perfect shade match if you only do the 2 front teeth.

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.