Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
I am not with whatever co. you have gotten these phones from... but I'd like to help if I can.
Please tell me what co. you get the phones from.
And what co. you get your phone line service from... if it is different.
And I suggest to start with, that if the fourth phone's addition caused problems, maybe you should take it out of the configuration, unplug it and disconnect it, and see if the others work, at least.