Do you own BLT? If so, you should make now an additional and separate 4 day reservation for the last 4 days of your trip. Then, at exactly 7 months out from that second reservation you will be able to modify it to Poly If Poly is open.
If you do not own BLT, then you cannot book 8 nights at BLT until tomorrow, unless you mean only a total of 7 nights, because at exactly 7 months out, you cannot reserve more than 7 nights.
As to changing the reservation now to all of your nights at BLT, that would assure you have all nights you want. But to change to a split reservation in four days will be a two step process if Poly is open. I am assuming you will need points from that longer BLT reservation to reserve the four days at Poly, That will require you to first modify the BLT reservation to the first four days by dropping the days you want for Poly. You will then have to make a new reservation for Poly separate from that modification. You will have a risk of losing the last four days of your vacation because between the time you modify the BLT reservation and attempt the Poly reservation, Poly could be filled by others seeking it.
Personally I would just reserve BLT now for all the nights you want and drop any idea of a split stay because of that risk of losing those last four days. Normally, for April time, I would suggest otherwise because Poly is typically open at 7 months out in April. However, the time you want appears to be part of the Star Wars race weekend and that changes everything because that is a time of very high DVC demand when there may be a real risk of Poly disappearing between the time you modify the BLT reservation to four days and then attempt the Poly reservation.