So here is the reasoning behind the rule of 4:
First the rule states that you can't book and travel for reservations outside of the DVC system in the last 4 months of your use year.
You CAN travel during the last 4 months of your use year so long as you book it before the last 4 months of your use year (the banking deadline).
You CAN book travel during the last 4 months of your use year so long as that travel is scheduled for next use year.
The reason is simple: Whenever we trade out, the points that represent a reservation that you could have made are used to create a reservation. Either for cash bookings (if you traded to
DCL or one of the other non-DVC resorts), or they create a reservation to be traded with RCI or II (or whatever the time share trading company at the time happens to be).
The points that you use to trade out have the same restrictions that you do when they are used for other purposes. So, that means that if they are put in to the cash reservation pool, then Disney has to book a cash reservation that falls within the use period of your points. If they are put in to the time share trading pool, then someone would have to book and stay at that reservation before your points would have expired.
So, lets for a minute, imagine what would happen if the rule of 4 didn't exist. Let's say that it's October 1st and you have a November Use Year. So, your points will expire on October 31st. You are also WAY past your banking deadline and all DVC resorts are booked solid. You then figure out that there is room at the Poly to use your points. So you use your 160 points at the Poly for a stay. Those points, that expire in 30 days, then go to the cash stay pool of points in order to "pay" for your POLY reservation. Disney would then have to book a last minute reservation at a DVC resort for a cash guest to complete their stay by the end of October. This would be a VERY difficult thing to accomplish.
Same thing for trading out to another time share company. You find out that there is availability in Hawaii and book it. The time share trading company then gets a reservation week that has to be booked and completed before the end of October. Not gonna happen, especially since there isn't any availability.
If you book before your banking deadline, then the points that you use to trade out can be banked in to next use year and they still have 4 months to book and complete a reservation while those points are still good. This just makes it realisticly possible that they can find a useful way to get "paid" for those points that you used to trade out.