We have 3 red sexlinks in our mixed flock and they didn't start laying until they were around 20-22 weeks old. You don't need to worry yet if only 1 of your 4 is laying, the rest will start when they are ready. Watch for their combs & wattles to get redder and for them to squat...those signs mean they are getting ready to begin their egg laying life.
Oh, and if you have one that likes to sit in a nesting box, sing loudly at the top of her lungs but not have an egg? Again, don't worry...she's just practicing (that happened to me with one of our Australorps and we wore ourselves out running into the coop looking for a non-existent egg). Chickens are funny like that!
Here is a really interesting article in the Learning Center you may enjoy reading, part of it is about egg development and laying.