It is common practice with the plural possessive form to drop the final s, but if one opts to retain it the result is four consecutive double letters in one word (Please tell me I am not the first to come up with this!).
The other popular word with three consecutive double letters, bookkeeper, can also be found in this form, ie bookkeepers’s, but of course in this case the four double letters are not consecutive. I have also seen subbookkeeper, although I am not sure if this is a true word. Or you could even go one further and have five consecutive double letters with subbookkeepperson, though I am certain this manufactured word does not really exist (except here!).