You won't get 5th night free if the are booked as separate reservations.
+1
True, so it may cost the points of an extra room as you will not get the 5th night free, however, the extra nights this year may get you platinum status for next year.
I've found that with plat status, it often saves a room with every stay, (if you need one for the adults and one for the children)
because the upgraded suites (at "W", Westin, etc..) usually include another room (with either a sofa bed or another bedroom, plus living room with sofa)
so that stay costs only the points/price of one room, yet, it's often similar to 2 sep hotel rooms.