Hi OP, you have been very honest in this thread, it's clear how hurt you are and you are right to be.
It's not a drunken snog that you are throwing an almost marriage away for, but the loss of trust. Your DP took your choice away from you about whether to remain in the relationship by lying and then concealing it from you for 4 years, which means you now feel you built your relationship on a lie.
On top of that, he has had 4 years to get over it, for you it's brand new.
Slightly different, but I'll share my experience with you anyway. I met DP at a party one night, we talked for a few hours, exchanged numbers and arranged to go on a date. He says he knew then he really, really liked me and wanted a relationship. I then went back to to my friends at the party (so I hadn't totally ditched them).
At the end of the night he snogged someone else
(I don't think they slept together, but can't be sure) apparently I was actually nearby during the kiss but I didn't see. It was with a girl he'd pointed out to me earlier on as he said she had been following him about all night. Anyway, I had no idea. I few days later we were texting and I asked (fishing) if he'd managed to escape the other girl, he said "Yeah, managed to give that one a body swerve". Satisfied, we arranged a date, got on great, fell in love, perfect relationship.
Back of my mind a bitwas that girl, I asked a couple of times if anything happened, he said no. Then much later, once we had moved in together etc, he admitted he actually snogged her, but didn't want to take the risk of telling me in case I didn't want to go out with him. I was livid.
Partly, he always claimed he 'knew' I was the one from our first conversation, so why was he missing someone else? Secondly, he took the option away from me about whether I wanted to get involved with someone who would do that. I asked, he lied, I got involved with him under false pretences in a way, and he didn't tell me until I was in too deep to be able to get out of it, at least without a lot of pain, and that was not fair.
If it helps, in mind of glad now that I didn't know, as I might not have gone out with him, then we wouldn't have what we have now. And it's not worth throwing it all away for.
What your DP did was worse, no doubt, but maybe just further down the scale of the same principle. Space will help you get your head clear.