Switching from one color to another
For changing the led color or making any other led to glow you have to put the led number instead of leds[led number]  in this case so your code will basically look like.
void loop() { 
 // Turn the LED on, then pause
 leds[0] = CRGB::Red;
 leds[1] = CRGB::Red;    
 leds[2] = CRGB::Red;
 leds[3] = CRGB::Red;
 FastLED.show();
 delay(5000);
 // Now turn the LED off, then pause
 leds[0] = CRGB::Green;
 leds[1] = CRGB::Green;
 leds[2] = CRGB::Green;    
 leds[3] = CRGB::Green;
 FastLED.show();
 delay(5000);
}

Hope this helps