I would like it to be a 4 wheeler because of stability issues with 3 wheelers.
I'm very much interested in seeing sketches photos or plans of your 4 wheeler.
Be careful not to focus on one advantage of a crank-drive, because that's just about it, except that the mass in the middle of the bike gives a slight handling advantage in some circumstances. Everything else is disadvantage: Cost, reliability, cost of replacement drive components, stranding if you get a problem with the drive train, crunchy gear shifting and increased frequency of shifting.
You can get enough power to climb anything with a 250w motor if you choose electrical components for the characteristics that you need.
One advantage of crank vs hub for the same jobs ie speed as well as trailer hill hauling/climbing ability, is you can use less Watts. You said so yourself.
How many amps would a legal Bafang CST rated 250W need, in order to match the Bafang BBS01B's climbing ability? Assume the BBS01B controller is limited to 15A.
I was going to convert my Dahon Espresso with a rear hub. I don't think I'll bother now. I'd get duffed up and stabbed by thugs, and my bike'd get "grabbed" on a hill.
You can run it at 48v and around 22A, It would blitz any bike/motor running at 36v and 15A on any road you're likely to encounter. There's also the Xiongda 2-speed, which is like a winch when you run it with 48v.
36V X 15A = 540W is going to be beaten by 48V X 22A = 1056W. With my cadence sensing BBS01B crank drive, I have both hill climbing ability and speed over flat road, something I'd need the apparently unavailable Xiongda Two Speed to achieve, as well as a large battery capable of discharging 22A continuously without bursting into flames. My 20" wheeled Dahon Helios P8 folding bike's chainstays are not suitable for the Xiongda, even if available.
I had to stretch my frame to 150mm to fit a Xiongda 2-speed motor.
Unfortunately, Xiongda stopped selling motors direct to end users. Since then, they seem to have gone a bit quiet and you can hardly find the 2-speed motors for sale anywhere except a very expensive German seller. You could try emailing them to see if they can help. Bonnie Xiong is one of the family owners and she's always been very helpful:
Bonnie@xiongdamotor.com.cn
If you want to try one of these motors, you need the special controller to go with it. I'd recommend the KT version rather than the Lishui if you get a choice. You get them from Bonnie too along with anything else you need. It used to be about £125 for the whole kit (bare motor).
I'm not saying the OP should use a mid-drive, but I do think it'd wise to design in compatibility for one.
P.S. After controller amperage limit was raised:
P.P.S. 75kg cargo (home grown heavy veg or water) + 15.5kg Homcom folding trailer + me 72kg + 22.41kg bike = 184.9kg was dragged up steep hills regularly by my bike with the BBS01B controller at 15A, with 52T >32T = 32.5 gear inches. My 20" driven rear wheel has a 23% torque advantage over a 26" wheel, where tyre meets tarmac. It'd climb faster hauling the same trailer now with the controller current limit raised to 20A = 720W, but I won't be adding more weight to the trailer because it handles too worryingly odd with any more than 75kg cargo. And because the rear wheel QR skewer might be yanked out of the dropouts again.