This is a product of too small of a feed wire in gauge. Its called voltage drop. It is why the headlights are dim too at idle. Also all the connectors offer up a resistance too.

With the engine off voltage at the dome light is at 11 volts +/- 1 volt and when you put the 4 ways on you may see the voltage drop to 9 - 10 volts and then when the 4 way bulbs are out it will wink to the 11 volts again. But start the car and now the altanator is pushing 14.5 volts and that extra 2 volts does make a difference.