@FranticFox,
Wedding planning seems to bring out all sorts of weird hidden things, eh?
You have been together a very long time, and it is particularly long considering how young you must be (under 30, I am guessing from reading between the lines). He sounds like he's got some FOMO going on. While it hurts, it's kind of better that it's happening now, rather than after the I do's.
A four-year engagement is a really long time, and you would be at about the five-year mark even with the current date. Leaving an open-ended 'date to be named later' extends that more and more of course. In addition, without a firm date, nobody can do any planning. You can't purchase apparel or flowers, you can't get say a restaurant reservation for a meal afterwards, you can't even book the JP without a date.
It sounds to me like the planning made it very real to him, and now he wants to go back to those thrilling days of yesteryear when there was no planning going on, and it was just an amorphous 'someday' without a hard date and commitment baked into it.
I am thinking you need to step back and give him some space. And that means not just to scrap the date for now but also to take a break from your relationship. As in, you both get to see other people and everything, as if you were 100% no longer in a relationship.
Except my suggestion to you is to have a firm end date for that. Call it, say, six months. So if it was today, April 25 of 2017, it would be October 25th so call it Halloween.
And that's when you have the 'come to Jesus' meeting. I hate ultimatums in relationships, personally. I think they are horribly unworkable, so I am not suggesting one. But I am suggesting having a frank talk. And you need to remind him that no one's getting any younger. If you want children, then there is a very real end date to your fertility, assuming you want to have children biologically and not adopt. And even if you don't want kids, between you, me, and the internet, it gets harder to find a mate the older you are - and women are hit harder in this area than men are.
Asking you to hang around forever, when marriage is important to you, is unfair to you. But asking him to marry without sowing some wild oats that it sounds like he has, can create a situation where he will want to do just that after the vows have been taken.
BTW, I also don't love people marrying their childhood sweethearts. It's a lovely idea in theory, and a lot of people have perfectly fine marriages, but it creates a fear of missing out. Also, I'm not who I was when I was 15 (and I wasn't like that when I was 21, either), and neither are you.
I hope it works out, somehow. And sometimes the best endgame is to part ways. But whatever happens, don't lose yourself in the process.