I think there is lesser difference between a 3 and a 4 then there is between a 2 and a 3. Three through 5 are similar positions really and the difference is only in calibre/seniority with the former taking priority. The 6 position is also similar but at point you start batting with the lower order and tail and thus he should be more of a finisher, even in Tests.
Bangladesh is a stronger team with Shakib al Hasan.
Bangladesh is a stronger team without Shakib al Hasan.