my bet is on there being insufficient current running to your 4 servos.
no i know that problem, it thats different when you us 3 servos at ones with a load on them they "give up" for about on sec, then they try to work again. I had this problem before but fixed it by using about 10 to 11 volts the run the arduino. I could not use the 12 volts because the arduino got to hot. but it runs nice now.