The Cantrips known chart for a level 1 cleric states you know 3 cantrips at level 1. The light domain cleric at level one gets the light cantrip if she doesn't already know it.
So, does a light domain cleric get 4 cantrips at level 1? Could I take 3 other cantrips and then be granted Light from my domain?
Yes, and this and Nature are the only Domains that gets an extra cantrip, so other clerics still have 3 at first level while Light and Nature Domains gets 3 plus Light / (Druid cantrip).
All the clerics get bonus cantrips as they grow up levels as you can see in the Player's Handbook (PHB) page 57, starting at 3 and reaching 5 at level 10th, the Light and Nature clerics will have his extra Light / (Druid cantrip) cantrips, so they are an exception between clerics.