Answer:
This situation means that resources were not being efficiently used.
If society managed to produce 1 more unit of X with the same resources and technology, this means that some resources were idle in the past, which causes inefficiency.
This also means that the combination 3 units of X and 4 units of Y is a point inside the PPF. However, we do not know if the combination 4 units of X and 4 units of Y is a point inside the PPF, or on the PPF, because there could be some other combination that could be even more efficient (for example 5 units of both X and Y with the same resources and technology).