I had crowns added to my top 10 front teeth 4 years ago and they are chipping. Should my dentist replace them for free?

Dr. Lerch has very good input below.  I would add that there are many factors to consider.  If you are grinding your teeth, then that really needs to be looked at.  That will cause porcelain to chip as well as natural teeth.  Grinding can be caused or exacerbated by stress.  There is also sleep disordered bruxism (grinding).  This can be a sign of more serious condition Sleep Apnea.  Look up signs of Sleep Apnea (or Sleep disordered Breathing) on the Google and see if you may need to look into this further.

Dental crowns often can last for many, many years.  In my office, I like to see 20 years or more.  However dental insurance companies will pay to replace a damaged, chipped, broken, or decayed (from underneath) crown when it has been in the mouth just over five years.  If for some reason a crown come unbonded, or uncemented or chipped within six months after placement, we would replace/recement/redo for free.  Not after four years.  Four years of normal use, of normal wear and tear is a good service. I would not even expect a discount after four years.  Good luck!