I may be in the minority here, but I would find it very suspicious if a partner of four years didn't share their phone with me. My parents have my phone password. I trust them not to abuse it. My sister and husband casually use my phone all the time and I use theirs.

Is this one extreme? Yes. But if that recent TikTok challenge is anything to go by, asking a long-term partner to borrow their phone isn't a huge request and anyone worth your trust in them isn't going to hesitate.

(And for god's sake, if you're not monogamous, stop claiming you are! There are many people who want and respect polyamory. Don't step on people's hearts and lie to them. Just be honest if you don't want to commit to one person and let them find someone else! OP, I'm sorry you went through this. There is no excuse for cheating.)