Just because something works for now does not make it right,or safe.
Volts x amps = watts,you do the math,take a 110 volt line & multiply it with the amp of the breaker which in this case is 20 amp & you get a total available wattage figure of 2,220 & thats max draw before deducting 10% for saftey margin,after that a 20 amp line can safely supply 2,020 watts safely.
With that being said a 15 amp line can safely provide 1,485 watts running 110 volts,the numbers change when running 220 but the formula to figure out max line load stays the same.
If your running 4 600 watt ballasts on a single 20 amp 110 volt line chances are that you are not feeding the ballasts enough power which will result in lost light,just because the light powers up does not mean its operating at max performance & the odds are your lights are not putting out as much light as they should be.
The way your running those lights is not safe & is screwing your plants out of lumens needed for photosynthsis but its your grow brother,all we can do is give you facts.