I don't want to derail the thread and turn this into a "You better tell the OBS!" thread, but of all reasons to avoid telling the OBS, fear of your WW getting angry and refusing to reconcile should not factor into the decision at all. If she can't handle her behavior being exposed to the other party that she has harmed, if she values her relationship and/or fond memories with the OM, and refuses to be held accountable for her actions then she is not a good candidate for reconciliation at all. Heck, if you tell OBS (without giving your WW a heads up) and she finds out right away, then you know immediately that she's still in contact with OM.
As for OM's marriage with his wife, it's just about as "fake" as yours is. I'm 99% sure that the OBS has observed odd behavior or changes to his personality over the past 4 years, as you have with your wife... only she might not have a clue what the hell is going on. It's ikely that she's raised her suspicions or concerns with OM, but keeps getting horribly manipulated and gaslit. Alternatively, she might know about the affair already, but OM has her convinced that you're an abusive psychopath behind closed doors (a common lie used by WHs who want to cover their asses and protect their OW) so telling you the truth would be dangerous to their family.
Although I generally don't like comparing infidelity to addiction, if there is one thing that cheaters and addicts have in common it's that they don't stop unless they've really suffered consequences for their actions. If they are coddled and enabled, if they don't aren't forced to make difficult changes, or if they're, at worst, rewarded for their behavior by their BS carrying the entire burden of infidelity and fighting to win them back, then they will never stop.
Lastly, kissing another woman while your wife is pregnant is despicable behavior, but her getting "revenge" on you certainly didn't help things... it just created more fodder for future resentment. Would she tolerate you having a 4-year affair to get back at her? My guess is no.
[This message edited by BluerThanBlue at 4:49 PM, Saturday, August 3rd]
BW, 40s
Divorced WH in 2015; now happily remarried
I edit my comments a lot for spelling, grammar, typos, etc.