This thread will probably rumble on for hours and OP doesn't seem to want to hear what the majority are saying. The basics are:
Sarah is not your best friend. She has been lying to you for 4 years.
Sarah has no conscience, she has been having an affair with her other "friend" Emma's husband.
Sarah doesn't care about either of you, despite how charming and lovely she appears to be, it's all an act.
Please consider why, after four years, she is telling you this now. She is trying to back John into a corner with you as leverage.
Please consider why she doesn't want your husband to know. That alone should ring alarm bells.
This is not a friendship group, it's just a convenient bunch of people to socialise with. Two of whom are having an affair, one is being gaslighted and betrayed by her friend and husband, one is now being pressed to stay silent about it in order to help them cover it up.
My parents were often in situations like this. Not them but the friendship group they were thrown together with (rural location, 1970s, not much else to do). A lot of affairs were going on, they always, always got found out and a lot of people got divorced.
My parents saw the light and distanced themselves from these toxic, vain, people. They never had many friends after that but saw it as a better option.
You need to have a word with your husband, OP.