Where to start... First of all, when determining what you are going to need as far as any electrical gear is concerned, you want to remember that you don't want to exceed 80% of rated capacity for any of your electrical parts and/or appliances - everything from plugs and receptacles to wire to inverters and generators. If you design your system so that your maximum load never exceeds 80% of rated capacity, you will much more trouble-free system, and all of your electrical stuff will last much longer and operate more trouble-free. With this in mind, a 4kW generator is too small, you need to step up to a 5kW. And, yeah, 4kW is real close at 3200 Watts, but you want to use the generator for six hours per day and that's a lot of use. Realistically, in your circumstances a 5kW unit would last a lot longer and give you more trouble-free operation.
You are asking a lot out of generators that are designed for running lights and charging cell phones. If you want to put a continuous load of six hours on a generator, day in and day out, you need a diesel workhorse, not a light-duty petrol battery charger.
If you are going to be looking at Chinese generators, you might as well buy direct from China and save about half. Alibaba is probably the easiest and safest way to buy stuff direct from China.
On the other hand, if you are looking for a "real" generator, take a look at these Listers on ebay(UK). Most of those are way bigger than what you need, but you can get an idea of the cost. There's one 10kW unit on that page for £1,500, but it's really not a very not a pretty specimen, although it gives you an idea of price. With something like this unit, you would have enough power to run your boat and your roaster for only £2,250. You'd probably have to go diesel/electric or hydraulic on the boat propulsion, but you'd have plenty of power to run battery chargers along with propulsion and roaster(s). You could add a few solar panels to your roasting boat and advertise yourself as the coffee company that cares about the environment. (Nothing wrong with good/free PR). You should also consider that, from what you have said about an 18 minute roasting cycle, you will be roasting about 15 - 20 kg per day. That's not a whole lot of coffee and, if you are successful, you will need to get more roasters, won't you? When deciding on your electrical system, you should consider what your future needs will be too.
Decisions, decisions.....
Whatever you decide to do, good luck with your project! I hope all works out well and that you are successful!