Just to clarify, you cannot book AND stay outside of the DVC resorts in the last 4 months of the UY. The non-DVC stay must at least be booked before you get within 4 mos of the end of your Use Year.
If you have a June UY, a non-DVC stay must be booked no later than January 31 to avoid falling within that 4 month window (Feb to May). The stay can still occur in those months, but the reservation must be secured prior to 1/31.
Disneyland resort stays are currently exempt from this rule.