I think the Genesis would be down around 1200 engine rpm to spin 540 pto shaft rpms. If they were an electronic pump, they might have more chance....but holding the rpms steady with a changing load (like a generator) would be a challenge.

I've always thought it too bad they didn't make a 750 rpm shaft adapter for such jobs.

I realize you're not as far north, but I'd hesitate running a larger tractor with a 100rpm shaft for the simple fact that the tractor wouldn't be running fast enough or pulling hard enough to create enough heat to keep from building up soot and carbon.  None of my newer tractors run even close to the green in the winter unless they are working moderately.  I run a Farmall 450 diesel on my generator and it runs very economically.  Running at approximately half throttle, it keeps the oil and coolant at a good temperature.  The governor response on that is second to none also.  Bear in mind that it's usually -20 or more windchills, sometimes down to -60 to -70 when our power goes out up here.  That's when the WD 45 comes out and runs on the generator until the tank heater on the 450 can do it's wonders.

The 8770 and 8870 Genesis tractors are some of the most fuel efficient tractors ever made. Trouble is, the only way you'll realize the full fuel efficiency of the 8770 is to load it to at least 75% of rated PTO HP, which would be a lot more than it takes to develop 20kW. At lower power ratings, I don't think you'll do all that much better than the 4000, and as others have pointed out, running the 8770 isn't cheap in terms of what a tach hour costs you on a trade-in or sale.

Also, consider this: The 4000 will probably go up in value in the future as an antique because it is in such nice condition, so you'll likely not even lose value on the 4000.

Oh, and should you put enough hours on the 4000 to warrant a rebuild -- it has to be easier to rebuild that cute little machine than an 8770.

Kelly, I think you are comparing 'too big' and 'too bigger'.  A 35kw generator needs about a 50 hp tractor, not 125 for the 'small' side of the comparison. 

I'm still thinking the 4000 is going to win.