Cosmetic Crowns Put in 3-4 Years Ago Have Always Hurt. I Found That the Margins Are Exposed. Why?

Without xrays to look at, there are too many possible reasons why, and most have nothing to do with negligence.  It simply is what it is.  The only real option LIKELY is to replace the crowns.

Any dental work, including many cosmetic dentistry treatments are still minor surgery. Most people should expect some discomfort after new dental work. BUT, it should not be too major, AND it should not last too long.

After cosmetic dental crowns, I would expect your teeth to feel generally comfortable in around a week, sometimes two weeks.

I tell all of my patients what to expect, and if they are having an issue longer than two weeks, I need to see them for follow up. 

In your case, the most important thing is to get your teeth comfortable. It could be doing some minor bonding at the margins of the crowns. A worst case scenario could mean removing a crown, inspecting under it, desensitizing the tooth, and possibly a new crown.

It's very important to have good communication with your cosmetic dentist. In a calm manner, ask a few direct questions and see if he can or is willing to take more action to get you comfortable.

See a very good experienced cosmetic dentist to replace the crowns. Remember, if you look for bargains, you get what you pay for!

Even the most experienced dentist can have an issue with crowns being seated improperly.  An experienced dentist, however, should take post-op x-rays after the crowns have been seated to make sure they are properly fit.  If they find that they have not been seated perfectly, a good dentist will rectify the situation for no charge so the patient has no future issues.  In your case, I would recommend contacting the original dentist that placed your crowns and see if they can offer any solutions to you.  If they are unwilling to fix the problem, the only other solution would be to get the crowns redone by a more experienced and honorable dentist with great references that won't leave you in the same predicament.