Hello OKfarmer,
With your 8/1, horsepower will be pretty much proportional to rpm in its normal operating range, so if you are cutting revs by less than 10%, the output will reduce by a similar amount. Depending on alternator and drive losses ,I assume you could still pull 4kw continuous. If you can maintain frequency and the exhaust smoke is only slight, you should be fine.
Regards, Combustor.