in a real case, i found that the result ( an aerosol species concentration) weren't different between applying 3 levels nesting and 4 levels nesting.
the inner grid spacing i set is 3.3km for 3 levels nesting.
the inner grid spacing i set is 1.1km for 4 levels nesting. but in this case, it spend more time consumption than last one.
but it is just one location.
i don't know in which case, the 4 levels nesting will much closer to the true (observation) than same case in 3 levels nesting.