Can anybody help me please, I have four female westies and they bark no consistently but they are annoying my neighbours

and its causing a big problem. They have access namely a dog flap so their are not looked outside they have full accessibility to kitchen and outdoors.
I have read about dogs collars that send out high pitched sounds etc but because I would have to buy four it works out expensive and I don't know if they work. I would not consider using the collar that gives off a shock should they carry on barking.
Any help would be much appreciated.