Leafbike has them. They try to push the 2000W "high power", but they also have the well-reviewed here 1500W "latest" available in 4-inch rim. They would probably be willing to lace their lower-power motors in there, too. Just ask them. I wouldn't do 2WD, I see it as the worst of both worlds, but hey if your heart is calling to do that, who am I to say naught. I think it is done for extreme sand circumstances, etc., but very rare, or maybe more as a show or attention-getting type of thing (I have seen vendors do it to put in promo videos, that's usually when I see it done that way). One thing I've always hated about internet forum culture (not necessarily here), is the tendency for people to attack the premise of a question (and not even answer the question), instead of just answering the question directly. In this case, I still think the premise is worthy of discussion, unless you are really sure of what you want, in which case, that's cool by me, it's your project, and you're not limited to one build anyway. And whatever happens, you will learn things (and be able to share).
EM3EV used to offer a 4-inch AlexRims Blizzerk, but I believe has gone to a 3-inch rim. I would've much preferred the narrower 3-inch rim they offer now, but I was an early buyer and got the Blizzerk. They may still be able to get one if you really want 4 inches, though. To me 4 inches are ridiculous wide. Before they settled on the 1st rim, I actually urged them to use a 50mm/2-inch rim, which is an extremely versatile rim size IMO. At the time for me, there was nothing available ebike-vendor-wise between standard mountain bike and ridiculously wide, and there still isn't much in-between. But that's not your question.
Neptronix bought a laced Leafbike wheel (made by "professional wheel builders" per their website), and it was so out of true, he had to get it professionally trued. On the other hand, my EM3EV one is amazingly true (from what I can tell). Meaning I spin the motor when the bike is upside-down, and that rim does. not. move. I think mine was the first they sent out, the owner was supervising, but still. They did a great job. They are in China but the owner is British, and lives on-site.
I know there is someone on this forum who owns a US outfit and at least a year ago they were making fatbike 4" rim kits with the heavy cromotor in it, the name escapes me right now. Not exactly up your alley, but giving brainstorming thoughts. Most kits sold for fatbikes are of the most powerful variety, as fatbikes and low-power [single] motors usually don't go well together.
Hope that helps a bit.
