You will see on this site everywhere....."TRUST YOUR GUT".
Just went back & read your posts from last summer saying the same things as now basically. My feeling is that this is still the same A from 2 years ago & it has just gone underground but you haven't trusted your gut feelings the whole time. They won't go away for a reason.....listen to them!!!!
Follow him yourself. Park outside his work on the weekend he wants to do overnight soon, & see what is really up. Have a friend do it, if you can't, etc. Find a way & I bet you get all your answers right there.
And as for "his said privacy need" ......basically, the only thing needed to respond to him about that is.........."you don't get privacy when you have had an A".
(Plus, privacy is when you want to close the bathroom door when you take a dump - without your phone present, of course. A secret phone is not privacy, it's secretive, plain & simple & for those that have nothing to hide....why a secret phone????)
Honey, you need to file for divorce, like NOW. It will either wake his ass up or you will be able to move on to a better life with your kids.
He will have to pay CS for 4 kids plus possible alimony, lose half of everything of his, etc., etc., etc. Sometimes being served is what it takes to get them to thinking about all the above mentioned & make definitive decisions about their A. Sometimes not & then you take half of everything, get your CS & SS & move on with your life. You will be happier, I guarantee it & love fades, believe me.
Sending hugs & strength your way!!!