A google search turned up a site that had this link on it. Here is a link to the site itself. http://www.smokstak.com/forum/showthread.php?t=49192
Below is a link to a shop manual for the engine that gives the HP. In answer to your question though the engine apears to be around 4.5 to 5 HP depending on the spec used while a 4000 watt generator will take closer to 5.5 HP to drive at the full 4000 watt output. Given that the engine is around 1/2 HP too small at the rated wattage it's not going to work since you need at least the full 5.5 HP available as well as a little extra to insure the engine can keep up and not be lugging at full load all the time. Good luck and I hope this helps.