If you really wanted to go down that route, and assuming it's an intermittent load, then fit some extra batteries and a BFO inverter.
When I was fixing our old Kangoo a while back, using current clamp meter to find faiked glow plug, I found that the alternator was managing about 50A at idle
Which I didn't expect. So you'd need to rig up a "fast idle" arrangementvto get the full 100A (or whatever) your alternator should be capable of. Perhap something a bit clever that presses the throttle a little when the battery voltage drops under load.
I wouldn't go down the "split charge" route unless youbare up for some major electrical reconfiguration. A "normal" split charge setup would disconnect your aux batteries (and hence your coffee machine) from the alternator just when you want it. To make a split charge work for a big loads like this would mean rewiring the alternator to the aux battery and making the vehicle battery into the secondary - not trivial, especially with modern monitoring/control computers etc.
There is also the issue of load dump transients. When your 4kW (i.e. 350 to 400A) load turns off, the alternator takes a finite time to turn itself down - and in the meantime it's stuffing out "more than is needed" which result in a spike in system voltage.
This sort of discussion comes up from time to time in Land Rover forums in relation to powering winches ...
An alternative is to fit a second alternator. If the engine is designed to run aircon but it isn't fitted, then fettling an alternator into the aircon pump space might not be too hard. You could also go 24V which would considerably reduce currents and the cable sizes needed.
