This would give you a terminal voltage of 25.9v (nominal) and about 29.4v (fully charged) and the first thing to do is check your ESC's and Motors can handle this power, which I doubt but not having knowledge of your set-up I should not comment any further on that
On an electrical point of view, there is nothing to stop 3S and 4S being used in series. The big difference in C rating and capacity (mAh ratings) will cause an uneven discharge rate between the batteries which you would need handling.
FWIW my suggestion would be to get/use the Graphene 4S 8000mAh 15C (120A burst) on the hex (two of the same in parallel if you need more flight time) and use the 3S for something else.
Hope this helps