While Bad Boys fans had wait nearly two decades between the second and third movies, there isn’t expected to be nearly that long of a time lag between the third and fourth movies.
That being said, since National Treasure 3’s Chris Bremner has only just started writing the script, unless he churns that thing out quickly, I imagine we’re in for at least a three year wait for Bad Boys 4, especially since it’ll take some doing to ensure Will Smith and Martin Lawrence’s schedules don’t conflict.